The rotating light on a lighthouse is 400 feet from a cliff. It completes one rotation every 10 seconds. The equation representing the distance, d, in feet that the center of the circle of light is from the lighthouse is d(t) = 400sec (πt/5). What is the period of d(t)? (Enter only the number.)

Answer 1

for y = cos kt
the period would be 2π/k
since the period of sec is the same as the period of sin

period = 2π/(π/5)
= 2π(5/π)
= 10

Didn't it tell you that at the beginning ?