### Home > CALC > Chapter 4 > Lesson 4.1.2 > Problem 4-25

4-25.

Area of a circle = *πr*^{2}

Sketch the radius and look for a geometric way to find its length.

Do you see the similar triangles?

Similar triangles have proportional sides, write a proportion to solve for *r*. Be careful! The height of the small triangle is not *h*.