Question

If cos x = -4/5, and pi < x < (3pi)/2, what is sin(x + pi/2)

please help and thank you :) xx

Answer 1

Try this solution:

note, that sin(pi/2+x)**=cosx, it means sin(x+pi/2)= -4/5.

ps/ ** may be used the formula sin(a+b)=sinacosb+sinbcosa.