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Question
Find the interval in which the following function are increasing or decreasing f(x) = 5x3 − 15x2 − 120x + 3 ?
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Solution
\[\text { When } \left( x - a \right)\left( x - b \right)>0 \text { with }a < b, x < a \text { or }x>b.\]
\[\text { When } \left( x - a \right)\left( x - b \right)<0 \text { with } a < b, a < x < b .\]
\[f\left( x \right) = 5 x^3 - 15 x^2 - 120x + 3\]
\[f'\left( x \right) = 15 x^2 - 30x - 120\]
\[ = 15 \left( x^2 - 2x - 8 \right)\]
\[ = 15 \left( x - 4 \right)\left( x + 2 \right)\]
\[\text { For }f(x) \text { to be increasing, we must have }\]
\[f'\left( x \right) > 0\]
\[ \Rightarrow 15 \left( x - 4 \right)\left( x + 2 \right) > 0 \]
\[ \Rightarrow \left( x - 4 \right)\left( x + 2 \right) > 0 \left[ \text { Since } 15 > 0, 15 \left( x - 4 \right)\left( x + 2 \right) > 0 \Rightarrow \left( x - 4 \right)\left( x + 2 \right) > 0 \right]\]
\[ \Rightarrow\text{ x }< - 2 \ or \ x > 4\]
\[ \Rightarrow x \in \left( - \infty , - 2 \right) \cup \left( 4, \infty \right)\]
\[\text { So },f(x)\text { is increasing on x } \in \left( - \infty , - 2 \right) \cup \left( 4, \infty \right).\]
\[\text { For }f(x) \text { to be decreasing, we must have },\]
\[f'\left( x \right) < 0\]
\[ \Rightarrow 15 \left( x - 4 \right)\left( x + 2 \right) < 0\]
\[ \Rightarrow \left( x - 4 \right)\left( x + 2 \right) < 0 \left[ \text { Since } 15 > 0, 15 \left( x - 4 \right)\left( x + 2 \right) < 0 \Rightarrow \left( x - 4 \right)\left( x + 2 \right) < 0 \right]\]
\[ \Rightarrow - 2 < x < 4\]
\[ \Rightarrow x \in \left( - 2, 4 \right)\]
\[\text { So, }f(x)\text { is decreasing on } x \in \left( - 2, 4 \right) .\]
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