If , where is an acute angle, then .
Suppose is a right-angled triangle. If , , and , then
the ratio of the lengths of the triangle's sides is
The cosine of an angle in a right-angled triangle is equal to the
ratio of the length of the adjacent side to the length of the
hypotenuse. The triangle shows that . Use this knowledge to help solve for as follows:
Thus, the value of is 15.