1.
`cos^2x-sin^2x=1-2sin^2x`
`VT=cos^2x-sin^2x=(1-sin^2x)-sin^2x=1-2sin^2x=VP` $\text{(đpcm)}$
2. (Đề sai)
`frac{1+sin^2x}{1-sin^2x}=1+tan^2x`
`VT=frac{1+sin^2x}{1-sin^2x}=\frac{1+(1-cos^2x)}{cos^2x}=\frac{2-cos^2x}{cos^2x}=2/cos^2x-1=1+2tan^2x`
3.
`3-4sin^2x=4cos^2x-1`
`VT=3-4sin^2x=3-4(1-cos^2x)=3-4+4cos^2x=4cos^2x-1=VP` $\text{(đpcm)}$
