Equivalent to square root of 60/ square root of 5?

Express the following as a function of a single angle. cos(60) cos(-20) - sin(60) sin(-20)

Bring the fraction a/a-4 to a denominator of 16-a^2

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?