Which Triangle Is It?
Part of the Triangles section of Coddy's Geometry journey. Lesson 39 of 60.
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.
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
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Triangles
1The Triangle Inequality
Can Three Sticks Meet?Exactly ReachingThe Range of the Third SideRecap: The Third Side5The Pythagorean Theorem
Squares on the SidesFinding the HypotenusePythagorean TriplesWhen It Is Not WholeRecap: The Hypotenuse8Special Right Triangles
The 45-45-90 TriangleLegs from the HypotenuseThe 30-60-90 TriangleAll Three SidesWhich Triangle Is It?Recap: Special Triangles3Perimeter
Around the EdgeA Missing SideIsosceles PerimeterSides as ExpressionsRecap: Around the Edge6Finding a Leg
Working BackwardsThe LadderWhich Side Is c?A Leg That Is Not WholeRecap: Finding a Leg