For x greater than or equal to zero and less than or equal to 2 pi, sin x and cos x are both decreasing on the intervals

a. (0 , pi/2)
b. (pi/2 , pi)
c. (pi , 3 pi / 2)
d. (3 pi / 2 , 2 pi)

step-by-step explanation:

no, her answer is not correct.   we square the lengths of all sides and then determine whether the pythagorean theorem holds here:

9² + 15² = 306.   this is not equal to the square of the longest side ( that square being 395.   answer c is correct.