For the simple harmonic motion equation d=9cos((pi/2)t) what is the period

For the simple harmonic motion equation d=9cos((pi/2)t), what is the period? 

A cosine function y = a cos bt + d has a period of (in terms of radians) P = 2π/b, where here the "b" value is π/2..thus plug π/2 for b in the period formula and evaluate.. 

2π/(π/2) <-- flip second fraction and multiply.. 

= 2π * 2/π <-- the π's cancel out..leaving us with just.. 

= 4 <-- answer... 

Thus, the period is 4.

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