Đáp án+Giải thích các bước giải:
a,
`cos^4 x-sin^4 x`
`=(cos^2 x+sin^2 x)(cos^2 x-sin^2 x)`
`=1.(cos^2 x-sin^2 x)`
`=cos^2 x-sin^2 x(đpcm)`
b,
`sin^4 x+cos^2 x.sin^2 x+sin^2 x`
`=sin^2 x(sin^2 x+cos^2 x +1)`
`=sin^2 x(1+1)`
`=2sin^2 x(đpcm)`
c,
`(1+tanx)(1+cotx)-2`
`=1+cotx+tanx+1-2`
`=cotx+tanx`
`=\frac{cosx}{sinx}+\frac{sinx}{cosx}`
`=\frac{cos^2 x+sin^2 x}{sinx.cosx}`
`=\frac{1}{sinx.cosx}(đpcm)`
