Trigonometry: finding value of sin(a)

Question

How to solve such problems:

a) find sin(a), when tan(a/2)=4/3 and 0<(a/2)<(π/2)

b) sin (4a), when tan(2a)=-2 and (π/2)<2a<π

Answer 1

a) Use half angle formula

b) Use double angle formula