Respuesta :

sin^2(x) = (1-cos2x)/2
sin 7pi/8 = rad((1-cos 7pi/4)/2)

cos(7pi/4)= cos(pi+3pi/4) = - cos 3pi/4 = 0

sin 7pi/8 = rad(2)/2

I hope that this is the answer that you were looking for and it has helped you.