#3(sin^4theta+cos^4theta)-2(sin^6theta+cos^6theta)#
#=3((sin^2theta+cos^2theta)^2-2sin^2thetacos^2theta)-(2(sin^2theta+cos^2theta)^3-3sin^2thetacos^2theta(sin^2theta+cos^2theta))#
#=3(1^2-2sin^2thetacos^2theta)-(2(1^3-3sin^2thetacos^2xx1)#
#=3-6sin^2thetacos^2theta-2+6sin^2thetacos^2#
#=1#