sin^6α+cos^6α+3sin^2αcos^2α
=(sin^2a)^3+(cos^2a)^3+3sin^2αcos^2α
=(sin^2a+cos^2a)(sin^4a-sin^2acos^2a+cos^4a)+3sin^2αcos^2α
=sin^4a-sin^2acos^2a+cos^4a+3sin^2αcos^2α
=sin^4a+2sin^2acos^2a+cos^4a
=(sin^2a+cos^2a)^2
=1