Question

Evaluate the two trigonometric functions using its period as an aid.

1. cos (9pi/2) = ?

Ans: 0

2. cos (-9pi/2) = ?

Ans: 0

Answer 1

once around is 4 pi/2
twice around is 8 pi/2

so this is twice around plus pi/2
which is pi/2
or
90 degrees

I suspect you can do the clockwise one now.

Answer 2

But isn't cos (pi/2) = 0, because cos(t) = x?