Đáp án + Giải thích các bước giải:
`cos^6\alpha+sin^6\alpha+3sin^2\alpha.cos^2\alpha`
`=cos^6\alpha+3sin^2\alpha.cos^2\alpha.1+sin^6\alpha`
`=cos^6\alpha+3sin^2\alpha.cos^2\alpha.(cos^2\alpha+sin^2\alpha)+sin^6\alpha`
`=cos^6\alpha+3sin^2\alpha.cos^4\alpha+3sin^4\alpha.cos^2\alpha+sin^6\alpha`
`=(cos^2\alpha)^3+3.(cos^2\alpha)^2 .sin^2\alpha+3.cos^2\alpha.(sin^2\alpha)^2+(sin^2\alpha)^3`
`=(cos^2\alpha+sin^2\alpha)^3`
`=1^3=1`