Show that if h, h': xy are homotopic and k, k': yz are homotopic then kh and k'h' are homotopic.

What percent of the circle below is shaded?

If the altitude is drawn to the hypotenuse of a right triangle, then the two triangles formed are similar to the original triangle and to each other. which set of triangles shows the similar triangles created by the altitude in the diagram?

Prove that 16^4-2^13-4^5 is divisible by 11.