trigonometry?

Question

find the exact values of sin 2u, cos2u, and tan 2u using the double-angle formulas:

Sec u = -(5/2)

pi/2 < u < pi

Answer 1

The cos is - 2/5. (Why?)

The sin is +/- the square root of 5 over 5. (Why?)

Of the two possibilities, it's the positive value. (Why?)

That should get you started. :)