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Which Triangle Is It?

Part of the Triangles section of Coddy's Geometry journey. Lesson 42 of 63.

A special triangle does not always announce itself with a square. Two angles are enough, because the third is 180 minus them. If two angles are 45° and 45°, the third is 90°: a 45-45-90 triangle. If they are 30° and 60°, the third is 90° too: a 30-60-90 triangle.

$Three triangles: a right angle and 50 degrees, 45 degrees and a right angle, two angles of 60 degrees$

Here the first triangle is right but not special: its other angle is 40°. The second has a right angle and 45°, so it is a 45-45-90 triangle. The third has two 60° angles, so its third angle is 60° and it is equilateral, not a right triangle at all. Half of it would be 30-60-90.

Challenge

Each triangle shows two of its angles. Work out each third angle, then tap the 30-60-90 triangle and press Check.

Try it yourself

$Find the right part on the figure$

Find the right part on the figure

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Triangles

2The Triangle Inequality

Can Three Sticks Meet?Exactly Reaching The Range of the Third Side The Formula Recap: The Third Side

5The Pythagorean Theorem

Squares on the Sides Finding the Hypotenuse Pythagorean Triples When It Is Not Whole Recap: The Hypotenuse

8Special Right Triangles

The 45-45-90 Triangle Legs from the Hypotenuse The 30-60-90 Triangle All Three SidesWhich Triangle Is It?Recap: Special Triangles

11Two Tools at Once

Height of an Isosceles Area from the Height The Equilateral Triangle Right Triangle Perimeter Recap: Two Tools