Question

If cosx=-3/5 and pi 12 years ago

Answer 1

cos x = -3 / 5
pi < x < 3pi / 2 -> x is in the third quadrant.
cos^2 (x / 2) = (1 + cos 2(x / 2)) / 2 = (1 + cos x) / 2
cos (x / 2) = -sqrt ((1 + cos x) / 2) = - sqrt ((1 - 3 / 5) / 2) = - sqrt (1 / 5) = -1 / sqrt 5 = - sqrt 5 / 5
It is negative because cosine x is negative in the third quadrant.

Answer 2

wow thanks!