Find the intervals on which each of the following functions is (a) increasing (b) decreasing.
f(x)=x4-8x3+22x2-24x+21
f’(x)=4x3-24x2+44x-24
= 4(x3 -6x2+11x-6)
=4(x-3)(x-1)(x-2)
Function f(x) is decreasing for x ∈ (-∞,1] ∪ [2, 3] and increasing in x ∈ (1,2) ∪ (3, ∞)
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