CK-12 Foundation
Ck 12: Geometry: Inscribed Similar Triangles
[Free Registration/Login may be required to access all resource tools.] Apply the fact that the altitude of a right triangle creates similar triangles.
Sophia Learning
Sophia: Using Corresponding Parts of Congruent Triangles
This lesson will demonstrate the use of CPCTC in triangle congruency proofs.
Mangahigh
Mangahigh: Shape: Proof of the Sum of the Angles in a Triangle
Determine that all angles in a triangle equal 180 degrees.
Mangahigh
Mangahigh: Shape: Proof of the Exterior Angle Theorem for a Triangle
Use your knowledge of exterior angles of a triangle to prove the theorem.
Texas Instruments
Texas Instruments: Proof of the Pythagorean Theorem Using Transformations
In this activity, students will explore visual proofs of the Pythagorean Theorem using area and transformations. They will use the Cabri Jr. app to perform translations on the squares constructed on the two legs and the hypotenuse of a...
Shodor Education Foundation
Shodor Interactivate: Squaring the Triangle
Learn about how the Pythagorean Theorem works through investigating the standard geometric proof.
University of California
University of California: Triangles and Simplices
This site from the University of California deals with different applications and topics pertaining to triangles, particularly triangulation.
Maths Challenge
Maths challenge.net: Circumscribed Triangle
Find the area of this circumscribed triangle with your geometry skills.
Maths Challenge
Maths challenge.net: Triangle in a Square
A "three star" geometry problem requiring some prior knowledge in math and proofs to maximize the area of an inscribed triangle in terms of the hypotenuse.
Science Buddies
Science Buddies: Circumscribing a Circle About a Triangle
Here is a project that combines Computer Science and Mathematics. Prove a method for circumscribing a circle about a triangle (as shown). You'll also learn how to create an interactive diagram to illustrate your proof, using an applet...
Science Buddies
Science Buddies: Inscribing a Circle in a Triangle Using the Geometry Applet
Here is a project that combines Computer Science and Mathematics. Prove a method for inscribing a circle within a triangle (as shown). You'll also learn how to create an interactive diagram to illustrate your proof, using an applet that...
Science Buddies
Science Buddies: Tangent Circles and Triangles
Here is a project that combines Computer Science and Mathematics. The two circles are tangent to one another at point A. Their diameters are parallel. Prove that points A, D and F are co-linear. You'll also learn how to create an...
Science Buddies
Science Buddies: Circles, Tangent Lines and Triangles
Here is a project that combines Computer Science and Mathematics. The semicircle has two tangent lines that meet at point T. You need to prove that a line drawn from A to T bisects CD. You'll also learn how to create an interactive...
National Council of Teachers of Mathematics
Nctm: Illuminations: Proof Without Words: Pythagorean Theorem
Students use this computer applet to visualize the Pythagorean Theorem.
Khan Academy
Khan Academy: Proofs With Transformations
Explore different ways of proving some theorems about lines and angles. Some transformations are used.
Paul Dawkins
Paul's Online Notes: Calculus I: Proof of Various Limit Properties
The tutorial proves some of the basic properties and facts about limits. Topics discussed are limit properties, triangle inequality, continuity, and limits at infinity. Class notes, definitions, and examples with detailed solutions are...
Maths Challenge
Maths challenge.net: Leaning Ladders
Use your right triangle solving skills to solve this ladders in a hole problem.
Maths Challenge
Maths challenge.net: Perpendicular Construction
This is a "three star" geometry problem where prior math problem skills and proof experience is recommended to solve this "Perpendicular Construction" problem.
Jim Loy
Jim Loy's Mathematics Page: Congruence of Triangles
Find explanations of the ways to prove triangles are congruent. There is a link for Congruence Of Triangles, Part II to find the proofs of some ways to prove congruence of triangles from other postulates (i.e. Assume SAS prove ASA).
Maths Challenge
Maths challenge.net: Measuring Mountains
Use your triangle solving skills to solve this triangle problem and then apply it to the real world.
Maths Challenge
Maths challenge.net: Right Angle Reasoning
A geometry problem for students to use their mathematical reasoning skills to explain a right triangle property.
Cuemath
Cuemath: Sine Law
This comprehensive guide explores the topic of sine law, by finding answers to questions like what is the law of sines, what are the applications of sine law, and proof of the sine law. Includes solved examples and practice questions.
Maths Challenge
Maths challenge.net: Shaded Triangle
A great geometry problem of area, encorperating many learned skills.
University of Georgia
University of Georgia: Perpendicular Bisectors Are Concurrent Proof
This site gives the proof that the three perpendicular bisectors of the sides of a triangle are concurrent.
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