Sin^4 x+ cos^4 x=sin x * cos xsin^4 x + 2sin^2 x * cos^2 x + cos^4 x = 2 sin^2 x cos^2 x + sinx*cos x(sin^2 x + cos^2 x)^2=2sinxcosx(2sinxcosx + 1)/22 = sin 2x * (sin 2x + 1) sin 2x = t, t in [-1,1]t^2+t-2=0t=1 sin 2x = 12x = pi/2 + 2*pi*k, k in Integersx = pi/4 + pi*k, k in Integers