The function $f(x)=2x^{3}-15x^{2}+36x-24$ is strictly decreasing in the interval is
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Logic Tip: For any upward-opening quadratic inequality $ax^2 + bx + c<0$ (where $a>0$) with real roots $r_1<r_2$, the solution is always the bounded interval $(r_1, r_2)$. You don't need to test numbers if you remember this geometric rule.