cos2a=√2÷3则sin^4a+cos^4a=sin^4(a)+cos^4(a) +2sin^2(a) * cos^2(a) -2sin^2(a) * cos^2(a)= [sin^2(a) + cos^2(a)]^2 - (1/2)[2sin(a)cos(a)]^2= 1- sin^2(2a) /2= 1- [1- cos^2(2a)] /2= 1- [1 -(√2÷3)^2] /...