Determine the monotonic intervals of the functions
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i need to determine the monotonic intervals of this function $y=2x^3-6x^2-18x-7$ . I tried the below but i am not sure if i am doing it right. My work: begin{align} y=2x^3-6x^2-18x-7 &Longleftrightarrow 6x^2-12x-18=0\ &Longleftrightarrow6(x^2-2x-3)=0\ &Longleftrightarrow(x-3) (x+1)\ &Longleftrightarrow x-3=0 x+1=0\ &Longleftrightarrow x=3, x=-1\ end{align} so my function increases when $xin[3, +infty[$ and decreases when $xin[-1, 3]cup ]-infty, -1]$ Please i want to know how to solve this problem any help with explanation will be appreciated. thanks in advanced
calculus functions
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