Pros: Interactive content wrapped in a simple package still delivers with tight, clear wording and digestible math chunks.
Cons: Ads can annoy and confusingly lead to other math sites.
Bottom Line: Free collection of math interactives and geometry construction tutorials.
Teachers can use Math Open Reference to easily project and step through constructions with the whole class. Next, students recreate them using the site until they are eventually able to do it with real drawing tools. Of course, when homework time rolls around, kids can peek back at the site if memory fails. Function explorers provide concrete illustrations of linear equations and more. Try letting students play with the sliders and see how changing different variables transforms the shape of the graph.
Continue reading Show lessMath Open Reference is a free interactive math reference covering high school geometry, functions, and calculus. Five organizing topics cover plane, coordinate, and solid geometry, plus trigonometry and calculus. A few function explorers and a general online scientific calculator arm kids with tools that help them visualize what all these numbers and formulas actually mean.
Most entries open with a definition, follow with concise explanations of relevant concepts, and then end with links to other relevant topics. Embedded "Try This" activities invite kids to interact with dynamic diagrams by moving intersecting points, lines, or vertices and watching the results. A large number of entries are actually constructions  stepbystep animations showing kids exactly how to find the "center of a circle using any righthanded object," for example.
Math Open Reference has some hidden gems despite its plain package and often confusing sidebar ads for other math sites. Concise, accurate explanations provide a solid foundation for a series of dynamic construction and manipulation applets that pull kids into the experience and build deeper, concrete understanding of geometry concepts. The constructions in particular show kids through animated protractors and pencils (and downloadable worksheets) how to bisect an angle, divide a line segment into equal parts, or draw a parallel line through a point.
While site owner John Page calls it a textbook in his mission statement, the terms reference and construction guide are more precise. Other sites featuring math simulations such as Desmos provide formative assessments that teachers can monitor; exercises with immediate feedback and a teacher dashboard are missing from Math Open Reference. Also missing is standards alignment, though many of the simulations align to the Common Core Math Standards.
Overall Rating
Engagement
Missing realworld context or games to make math interesting. The site isn't visually appealing, but easy to navigate.
Pedagogy
Concepts are well defined with clear language. The applets and "function explorers" provide visual representations of math concepts.
Support
Content is offered in all of the languages offered through Google Translate, including Esperanto, Haitian Creole, and Basque. Additional opportunities to extend student knowledge are needed.
Key Standards Supported
Building Functions
 HSF.BF.1
Write a function that describes a relationship between two quantities.
 HSF.BF.1.a
Determine an explicit expression, a recursive process, or steps for calculation from a context.
 HSF.BF.1.b
Combine standard function types using arithmetic operations. For example, build a function that models the temperature of a cooling body by adding a constant function to a decaying exponential, and relate these functions to the model.
 HSF.BF.1.c
(+) Compose functions. For example, if T(y) is the temperature in the atmosphere as a function of height, and h(t) is the height of a weather balloon as a function of time, then T(h(t)) is the temperature at the location of the weather balloon as a function of time.
 HSF.BF.2
Write arithmetic and geometric sequences both recursively and with an explicit formula, use them to model situations, and translate between the two forms.
 HSF.BF.3
Identify the effect on the graph of replacing f(x) by f(x) + k, k f(x), f(kx), and f(x + k) for specific values of k (both positive and negative); find the value of k given the graphs. Experiment with cases and illustrate an explanation of the effects on the graph using technology. Include recognizing even and odd functions from their graphs and algebraic expressions for them.
 HSF.BF.4
Find inverse functions.
 HSF.BF.4.a
Solve an equation of the form f(x) = c for a simple function f that has an inverse and write an expression for the inverse. For example, f(x) =2 x3 or f(x) = (x+1)/(x–1) for x ≠ 1.
 HSF.BF.4.b
(+) Verify by composition that one function is the inverse of another.
 HSF.BF.4.c
(+) Read values of an inverse function from a graph or a table, given that the function has an inverse.
 HSF.BF.5
(+) Understand the inverse relationship between exponents and logarithms and use this relationship to solve problems involving logarithms and exponents.
Circles
 HSG.C.5
Derive using similarity the fact that the length of the arc intercepted by an angle is proportional to the radius, and define the radian measure of the angle as the constant of proportionality; derive the formula for the area of a sector.
 HSG.C.1
Prove that all circles are similar.
 HSG.C.2
Identify and describe relationships among inscribed angles, radii, and chords. Include the relationship between central, inscribed, and circumscribed angles; inscribed angles on a diameter are right angles; the radius of a circle is perpendicular to the tangent where the radius intersects the circle.
 HSG.C.3
Construct the inscribed and circumscribed circles of a triangle, and prove properties of angles for a quadrilateral inscribed in a circle.
 HSG.C.4
(+) Construct a tangent line from a point outside a given circle to the circle.
Congruence
 HSG.CO.1
Know precise definitions of angle, circle, perpendicular line, parallel line, and line segment, based on the undefined notions of point, line, distance along a line, and distance around a circular arc.
 HSG.CO.2
Represent transformations in the plane using, e.g., transparencies and geometry software; describe transformations as functions that take points in the plane as inputs and give other points as outputs. Compare transformations that preserve distance and angle to those that do not (e.g., translation versus horizontal stretch).
 HSG.CO.3
Given a rectangle, parallelogram, trapezoid, or regular polygon, describe the rotations and reflections that carry it onto itself.
 HSG.CO.4
Develop definitions of rotations, reflections, and translations in terms of angles, circles, perpendicular lines, parallel lines, and line segments.
 HSG.CO.5
Given a geometric figure and a rotation, reflection, or translation, draw the transformed figure using, e.g., graph paper, tracing paper, or geometry software. Specify a sequence of transformations that will carry a given figure onto another.
 HSG.CO.12
Make formal geometric constructions with a variety of tools and methods (compass and straightedge, string, reflective devices, paper folding, dynamic geometric software, etc.). Copying a segment; copying an angle; bisecting a segment; bisecting an angle; constructing perpendicular lines, including the perpendicular bisector of a line segment; and constructing a line parallel to a given line through a point not on the line.
 HSG.CO.13
Construct an equilateral triangle, a square, and a regular hexagon inscribed in a circle.
 HSG.CO.10
Prove theorems about triangles. Theorems include: measures of interior angles of a triangle sum to 180°; base angles of isosceles triangles are congruent; the segment joining midpoints of two sides of a triangle is parallel to the third side and half the length; the medians of a triangle meet at a point.
 HSG.CO.11
Prove theorems about parallelograms. Theorems include: opposite sides are congruent, opposite angles are congruent, the diagonals of a parallelogram bisect each other, and conversely, rectangles are parallelograms with congruent diagonals.
 HSG.CO.9
Prove theorems about lines and angles. Theorems include: vertical angles are congruent; when a transversal crosses parallel lines, alternate interior angles are congruent and corresponding angles are congruent; points on a perpendicular bisector of a line segment are exactly those equidistant from the segment’s endpoints.
 HSG.CO.6
Use geometric descriptions of rigid motions to transform figures and to predict the effect of a given rigid motion on a given figure; given two figures, use the definition of congruence in terms of rigid motions to decide if they are congruent.
 HSG.CO.7
Use the definition of congruence in terms of rigid motions to show that two triangles are congruent if and only if corresponding pairs of sides and corresponding pairs of angles are congruent.
 HSG.CO.8
Explain how the criteria for triangle congruence (ASA, SAS, and SSS) follow from the definition of congruence in terms of rigid motions.
Expressing Geometric Properties With Equations
 HSG.GPE.1
Derive the equation of a circle of given center and radius using the Pythagorean Theorem; complete the square to find the center and radius of a circle given by an equation.
 HSG.GPE.2
Derive the equation of a parabola given a focus and directrix.
 HSG.GPE.3
(+) Derive the equations of ellipses and hyperbolas given the foci, using the fact that the sum or difference of distances from the foci is constant.
 HSG.GPE.4
Use coordinates to prove simple geometric theorems algebraically. For example, prove or disprove that a figure defined by four given points in the coordinate plane is a rectangle; prove or disprove that the point (1, √3) lies on the circle centered at the origin and containing the point (0, 2).
 HSG.GPE.5
Prove the slope criteria for parallel and perpendicular lines and use them to solve geometric problems (e.g., find the equation of a line parallel or perpendicular to a given line that passes through a given point).
 HSG.GPE.6
Find the point on a directed line segment between two given points that partitions the segment in a given ratio.
 HSG.GPE.7
Use coordinates to compute perimeters of polygons and areas of triangles and rectangles, e.g., using the distance formula.
Geometry
 6.G.1
Find the area of right triangles, other triangles, special quadrilaterals, and polygons by composing into rectangles or decomposing into triangles and other shapes; apply these techniques in the context of solving realworld and mathematical problems.
 6.G.2
Find the volume of a right rectangular prism with fractional edge lengths by packing it with unit cubes of the appropriate unit fraction edge lengths, and show that the volume is the same as would be found by multiplying the edge lengths of the prism. Apply the formulas V = l w h and V = b h to find volumes of right rectangular prisms with fractional edge lengths in the context of solving realworld and mathematical problems.
 6.G.3
Draw polygons in the coordinate plane given coordinates for the vertices; use coordinates to find the length of a side joining points with the same first coordinate or the same second coordinate. Apply these techniques in the context of solving realworld and mathematical problems.
 6.G.4
Represent threedimensional figures using nets made up of rectangles and triangles, and use the nets to find the surface area of these figures. Apply these techniques in the context of solving realworld and mathematical problems.
 7.G.1
Solve problems involving scale drawings of geometric figures, including computing actual lengths and areas from a scale drawing and reproducing a scale drawing at a different scale.
 7.G.2
Draw (freehand, with ruler and protractor, and with technology) geometric shapes with given conditions. Focus on constructing triangles from three measures of angles or sides, noticing when the conditions determine a unique triangle, more than one triangle, or no triangle.
 7.G.3
Describe the twodimensional figures that result from slicing three dimensional figures, as in plane sections of right rectangular prisms and right rectangular pyramids.
 7.G.4
Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.
 7.G.5
Use facts about supplementary, complementary, vertical, and adjacent angles in a multistep problem to write and solve simple equations for an unknown angle in a figure.
 7.G.6
Solve realworld and mathematical problems involving area, volume and surface area of two and threedimensional objects composed of triangles, quadrilaterals, polygons, cubes, and right prisms.
 8.G.9
Know the formulas for the volumes of cones, cylinders, and spheres and use them to solve realworld and mathematical problems.
 8.G.6
Explain a proof of the Pythagorean Theorem and its converse.
 8.G.7
Apply the Pythagorean Theorem to determine unknown side lengths in right triangles in realworld and mathematical problems in two and three dimensions.
 8.G.8
Apply the Pythagorean Theorem to find the distance between two points in a coordinate system.
 8.G.1
Verify experimentally the properties of rotations, reflections, and translations:
 8.G.1.a
Lines are taken to lines, and line segments to line segments of the same length.
 8.G.1.b
Angles are taken to angles of the same measure.
 8.G.1.c
Parallel lines are taken to parallel lines.
 8.G.2
Understand that a twodimensional figure is congruent to another if the second can be obtained from the first by a sequence of rotations, reflections, and translations; given two congruent figures, describe a sequence that exhibits the congruence between them.
 8.G.3
Describe the effect of dilations, translations, rotations, and reflections on twodimensional figures using coordinates.
 8.G.4
Understand that a twodimensional figure is similar to another if the second can be obtained from the first by a sequence of rotations, reflections, translations, and dilations; given two similar two dimensional figures, describe a sequence that exhibits the similarity between them.
 8.G.5
Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angleangle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.
Interpreting Functions
 HSF.IF.7
Graph functions expressed symbolically and show key features of the graph, by hand in simple cases and using technology for more complicated cases.
 HSF.IF.7.a
Graph linear and quadratic functions and show intercepts, maxima, and minima.
 HSF.IF.7.b
Graph square root, cube root, and piecewisedefined functions, including step functions and absolute value functions.
 HSF.IF.7.c
Graph polynomial functions, identifying zeros when suitable factorizations are available, and showing end behavior.
 HSF.IF.7.d
(+) Graph rational functions, identifying zeros and asymptotes when suitable factorizations are available, and showing end behavior.
 HSF.IF.7.e
Graph exponential and logarithmic functions, showing intercepts and end behavior, and trigonometric functions, showing period, midline, and amplitude.
 HSF.IF.8
Write a function defined by an expression in different but equivalent forms to reveal and explain different properties of the function.
 HSF.IF.8.a
Use the process of factoring and completing the square in a quadratic function to show zeros, extreme values, and symmetry of the graph, and interpret these in terms of a context.
 HSF.IF.8.b
Use the properties of exponents to interpret expressions for exponential functions. For example, identify percent rate of change in functions such as y = (1.02)t, y = (0.97)t, y = (1.01)12t, y = (1.2)t/10, and classify them as representing exponential growth or decay.
 HSF.IF.9
Compare properties of two functions each represented in a different way (algebraically, graphically, numerically in tables, or by verbal descriptions). For example, given a graph of one quadratic function and an algebraic expression for another, say which has the larger maximum.
 HSF.IF.4
For a function that models a relationship between two quantities, interpret key features of graphs and tables in terms of the quantities, and sketch graphs showing key features given a verbal description of the relationship. Key features include: intercepts; intervals where the function is increasing, decreasing, positive, or negative; relative maximums and minimums; symmetries; end behavior; and periodicity.
 HSF.IF.5
Relate the domain of a function to its graph and, where applicable, to the quantitative relationship it describes. For example, if the function h(n) gives the number of personhours it takes to assemble n engines in a factory, then the positive integers would be an appropriate domain for the function.
 HSF.IF.6
Calculate and interpret the average rate of change of a function (presented symbolically or as a table) over a specified interval. Estimate the rate of change from a graph.
 HSF.IF.1
Understand that a function from one set (called the domain) to another set (called the range) assigns to each element of the domain exactly one element of the range. If f is a function and x is an element of its domain, then f(x) denotes the output of f corresponding to the input x. The graph of f is the graph of the equation y = f(x).
 HSF.IF.2
Use function notation, evaluate functions for inputs in their domains, and interpret statements that use function notation in terms of a context.
 HSF.IF.3
Recognize that sequences are functions, sometimes defined recursively, whose domain is a subset of the integers. For example, the Fibonacci sequence is defined recursively by f(0) = f(1) = 1, f(n+1) = f(n) + f(n1) for n ≥ 1.
Linear, Quadratic, And Exponential Models
 HSF.LE.1.a
Prove that linear functions grow by equal differences over equal intervals, and that exponential functions grow by equal factors over equal intervals.
 HSF.LE.1.b
Recognize situations in which one quantity changes at a constant rate per unit interval relative to another.
 HSF.LE.1.c
Recognize situations in which a quantity grows or decays by a constant percent rate per unit interval relative to another.
 HSF.LE.2
Construct linear and exponential functions, including arithmetic and geometric sequences, given a graph, a description of a relationship, or two inputoutput pairs (include reading these from a table).
 HSF.LE.3
Observe using graphs and tables that a quantity increasing exponentially eventually exceeds a quantity increasing linearly, quadratically, or (more generally) as a polynomial function.
 HSF.LE.4
For exponential models, express as a logarithm the solution to abct =dwherea,c,anddarenumbersandthebasebis2,10,ore; evaluate the logarithm using technology.
 HSF.LE.5
Interpret the parameters in a linear or exponential function in terms of a context.
Modeling With Geometry
 HSG.MG.1
Use geometric shapes, their measures, and their properties to describe objects (e.g., modeling a tree trunk or a human torso as a cylinder).
 HSG.MG.2
Apply concepts of density based on area and volume in modeling situations (e.g., persons per square mile, BTUs per cubic foot).
 HSG.MG.3
Apply geometric methods to solve design problems (e.g., designing an object or structure to satisfy physical constraints or minimize cost; working with typographic grid systems based on ratios).
Similarity, Right Triangles, And Trigonometry
 HSG.SRT.10
(+) Prove the Laws of Sines and Cosines and use them to solve problems.
 HSG.SRT.11
(+) Understand and apply the Law of Sines and the Law of Cosines to find unknown measurements in right and nonright triangles (e.g., surveying problems, resultant forces).
 HSG.SRT.9
(+) Derive the formula A = 1/2 ab sin(C) for the area of a triangle by drawing an auxiliary line from a vertex perpendicular to the opposite side.
 HSG.SRT.6
Understand that by similarity, side ratios in right triangles are properties of the angles in the triangle, leading to definitions of trigonometric ratios for acute angles.
 HSG.SRT.7
Explain and use the relationship between the sine and cosine of complementary angles.
 HSG.SRT.8
Use trigonometric ratios and the Pythagorean Theorem to solve right triangles in applied problems.
 HSG.SRT.4
Prove theorems about triangles. Theorems include: a line parallel to one side of a triangle divides the other two proportionally, and conversely; the Pythagorean Theorem proved using triangle similarity.
 HSG.SRT.5
Use congruence and similarity criteria for triangles to solve problems and to prove relationships in geometric figures.
 HSG.SRT.1
Verify experimentally the properties of dilations given by a center and a scale factor:
 HSG.SRT.1.a
A dilation takes a line not passing through the center of the dilation to a parallel line, and leaves a line passing through the center unchanged.
 HSG.SRT.1.b
The dilation of a line segment is longer or shorter in the ratio given by the scale factor.
 HSG.SRT.2
Given two figures, use the definition of similarity in terms of similarity transformations to decide if they are similar; explain using similarity transformations the meaning of similarity for triangles as the equality of all corresponding pairs of angles and the proportionality of all corresponding pairs of sides.
 HSG.SRT.3
Use the properties of similarity transformations to establish the AA criterion for two triangles to be similar.
Trigonometric Functions
 HSF.TF.1
Understand radian measure of an angle as the length of the arc on the unit circle subtended by the angle.
 HSF.TF.2
Explain how the unit circle in the coordinate plane enables the extension of trigonometric functions to all real numbers, interpreted as radian measures of angles traversed counterclockwise around the unit circle.
 HSF.TF.3
(+) Use special triangles to determine geometrically the values of sine, cosine, tangent for π/3, π/4 and π/6, and use the unit circle to express the values of sine, cosine, and tangent for π–x, π+x, and 2π–x in terms of their values for x, where x is any real number.
 HSF.TF.4
(+) Use the unit circle to explain symmetry (odd and even) and periodicity of trigonometric functions.
 HSF.TF.5
Choose trigonometric functions to model periodic phenomena with specified amplitude, frequency, and midline.
 HSF.TF.6
(+) Understand that restricting a trigonometric function to a domain on which it is always increasing or always decreasing allows its inverse to be constructed.
 HSF.TF.7
(+) Use inverse functions to solve trigonometric equations that arise in modeling contexts; evaluate the solutions using technology, and interpret them in terms of the context.
 HSF.TF.8
Prove the Pythagorean identity sin2(θ) + cos2(θ) = 1 and use it to calculate trigonometric ratios.
 HSF.TF.9
(+) Prove the addition and subtraction formulas for sine, cosine, and tangent and use them to solve problems.
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