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Find the intervals in which f(x)=3/2x^4-...

Find the intervals in which `f(x)=3/2x^4-4x^3-45 x^2+51`

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`f(x)=6x^3-12x^2-90x`
`=6x(x-5)(x-3)`
`f'(x)=0`
`f'(x0>0,` for all `x in (-3, 0) cup (5, oo)` then `f(x)` is strictly increasing
`f'(x)<0` for all `x in (-oo, -3) cup (0, 5)` the `f(x)` is strictly decreasing.
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