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Could anyone help me solve this. I need to find the increase and decrease intervals of f(x) = sin(-3x).

The increase intervals of sin(x) are [itex]-\pi/2 + 2\pi*k < x < \pi/2 + 2\pi*k[/itex]

Is the following the right way to solve my problem (for increase intervals)? [itex]-\pi/2 + 2\pi*k < -3x < \pi/2 + 2\pi*k[/itex]

By multiplying by -3 you get: [itex]-\pi/6 + 2/3\pi*k < x < \pi/6 + 2/3\pi*k[/itex] (1)

Although when plotting sin(-3x) this interval (1) is the decrease one. Why?