Question 916775
A ball on a string is pulled 6 inches below its rest position and then released. After t seconds, the ball's distance (d) in inches from its rest position is given by

d = -6cos (pi/2)t

find all values of t for which the ball is 3 inches below its rest position.
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I think you mean
d = -6cos(t*pi/2)
-6cos(t*pi/2) = -3
cos(t*pi/2) = 1/2
t*pi/2 = pi/3, 5pi/3 + n*2pi, n = 0,1,2,3...
t = 2/3, 10/3 + 4n, n = 0,1,2,3... t in seconds