>>351
cosθ=c,sinθ=sとかく
2(c^6+s^6)-3(c^4+s^4)
=2{(c^2+s^2)^3-3(s^2+c^2)(cs)^2}-3{(c^2+s^2)^2-2(cs)^2}
=2{1-3(cs)^2}-3{1-2(cs)^2}=2-3=-1