2cos^2(3π/2-x/4)-2=0

cos^2 x\2 =1-sin^2 x\2
sin^2 x\2 -(1-sin^2 x\2 )=sin^2 x\2-1+sin^2 x\2=2sin^2 x\2-1=2(1-cosx)/2-1=1-cos x-1=-cos x