Question

In two or more complete sentences, explain why
cos(30°) + cos(60°) ≠ cos(90°)

Answer 1

By using the 30-60-90 right triangle, or the unit circle, you can find (without a calculator) that

$\cos30^o=\sqrt{3} /2$

$\cos60^o=1/2$

$\cos90^o=0$

Thus, cos(30°) + cos(60°) > cos(90°) and they are not equal to each other.