`1.`
`VT=cos^2 x - sin^2 x`
`=cos^2 x + sin^2x - 2sin^2 x`
`=1-2sin^2 x = VP`
`to đpcm`
`2.`
`3-4 sin^2 x=4cos^2 x-1`
`<=> 3-4sin^2 x - 4cos^2 x+1=0`
`<=> 4-4(sin^2 x+cos^2 x)=0`
`<=> 4(sin^2 x+cos^2 x)=4`
`<=> sin^2 x + cos^2x=1` ( Luôn đúng )
`to đpcm`