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`7)`

`cos(x+y)cos(x-y)`

`=1/2(cos2x+cos2y)`

`=1/2(1-2sin^2x + 1-2sin^2y)`

`=1 - sin^2x - sin^2y`

`=cos^2x - sin^2y` (đpcm)

`8)`

`3/8 - 1/2cos2x + 1/8cos4x`

`=3/8 - 1/2(1 - 2sin^2x)+1/8(1 - 2sin^2 2x)`

`=-1/4sin^2 2x + sin^2x`

`=-sinx^2cos^2x+sin^2x`

`=sin^2x(cos^2x-1)`

`=sin^2x*sin^2x`

`=sin^4x` (đpcm)