If $\sin A = 4/5$ with $A$ in QII, find $\cos A/2$

Question

For the following, assume that all the given angles are in simplest form, so that if A is in QIV you may assume that 270° < A < 360°.

If $\sin A = 4/5$ with A in QII, find $\cos A/2$

I keep getting $\cos \frac A2=-\frac{\sqrt5}5$, which I think is incorrect.

Answer 1

Cos(A/2) = sqrt((1+cosA)/2)

= sqrt(1-(3/5)/2)

= sqrt(1/5)

= 1/(sqrt(5))

= ( sqrt(5))/5