1、已知sinx^4+cosx^4=5/9,求sin2x的值 2、已知cos2x=3/5,求sinx^4+cosx^4

(sinx)^2 +(cosx)^2 =1 两边平方(sinx)^4 +(cosx)^4 +2(sinx)^2(cosx)^2 =1=> 5/9+2(sinx)^2(cosx)^2 =1 => (sinx)^2(cosx)^2 =2/9 sin2x =2sinxcosx => (sin2x)^2=4(sinx)^2(cosx)^2=8/9 => sin2x=±2√2 / 3 .(1)(...