Đáp án:

`y=2cos(3x-pi/4)`

 `->y'=[2cos(3x-pi/4)]^' `

`=-2sin(3x-pi/4).(3x-pi/4)^' `

`=-2sin(3x-pi/4).3 `

`=-6sin(3x-pi/4) `

`->y''=[-6sin(3x-pi/4)]^'`

`=-6cos(3x-pi/4).(3x-pi/4)^'`

`=-6cos(3x-pi/4).3`

`=-18cos(3x-pi/4)`

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