Giải thích các bước giải:
`cos^4x + sin^4x = cos2x `
`=>(cos^2x+sin^2x)^2-2(sinxcosx)^2=cos2x `
`=>1-2(sinxcosx)^2=cos2x `
`=>1-cos2x=2(sinxcosx)^2 `
`=>2sin^2x=2(sinxcosx)^2 `
`=>2sin^2x(1-cos^2x)=0`
`=>`\(\left[ \begin{array}{l}\text{sin}^2x=0\\\text{cos}^2x=1\end{array} \right.\) `=>`\(\left[ \begin{array}{l}x=k\pi\\x=k2\pi\end{array} \right.\)
