determine the maximum r values of r=7cos 40

Question

determine the maximum r values of r=7cos 40 this is one of my problems doing teacher said. I am wanting to pass my tests.

Answer 1

r = 7 cos 4t

dR/dt = - 28 sin 4t

if this derivative is zero, then 4t= 0 or 4t = pi

t= 0 or t = pi/4

the critical points are (0,7) and (pi/4, 7)