If sin A = - 3/5 with A in QIV , find cos(A/2)

Answer 1

The terminal point is at (4,-3)
cosA = 4/5, so
2cos^2(A/2) - 1 = 4/5
and since 3π/2 < A < 2π,
3π/4 < A/2 < π
That is, A/2 is in QII, so it is negative.