With every triangle, no matter what shape, the sum of its three angles will always be 180°. You have discovered this about the two different Isotiles. It is true that in every triangle that a + b + c = 180°.

Now we are going to think about a special sort of triangle.

A triangle is called isosceles if it has two sides of equal length. Here AB = AC. With any isosceles triangle, the angles opposite the equal sides are also equal. Thus the angle ABC is equal to the angle ACB. It works both ways. If you know that two angles of a triangle are equal, the sides opposite these two angles are bound to be equal.

Both our Isotiles are isosceles. You can measure the two equal sides, and the two equal angles. You have worked out how big each is.