Verified answer

(sin^4(a)+cos^4(a))^2-2sin^4(а)cos^4(а) = Sin^8(a)+ 2sin^4(a)cos^4(a)

+ cos^8(a) - 2sin^4(a)cos^4(a) =  Sin^8(a) + cos^8(a)

(a + b)^2 = a^2 + 2ab + b^2