原式＝[1-(sin^2x+con^2x)^2-2sin^2x*con^2x]/[1-(sin^2x+con^2x)*(sin^4x+con^4x-sin^2x*con^2x)]=(2sin^2x*con^2x)/{1-[(sin^2x+con^2x)^2-2sin^2x*con^2x-sin^2x*con^2x]}=(2sin^2x*con^2x)/(3sin^2x*con^2x)=2/3...