JEE MAIN - Mathematics (2022 - 25th July Morning Shift - No. 7)
The curve $$y(x)=a x^{3}+b x^{2}+c x+5$$ touches the $$x$$-axis at the point $$\mathrm{P}(-2,0)$$ and cuts the $$y$$-axis at the point $$Q$$, where $$y^{\prime}$$ is equal to 3 . Then the local maximum value of $$y(x)$$ is:
$$\frac{27}{4}$$
$$\frac{29}{4}$$
$$\frac{37}{4}$$
$$\frac{9}{2}$$
Explanation
$f(x)=y=a x^{3}+b x^{2}+c x+5 \quad \ldots$ (i)
$$ \frac{d y}{d x}=3 a x^{2}+2 b x+c \quad \ldots (ii) $$
Touches $x$-axis at $P(-2,0)$
$\left.\Rightarrow y\right|_{x=-2}=0 \Rightarrow-8 a+4 b-2 c+5=0 \quad \ldots ...(iii)$
Touches $x$-axis at $P(-2,0)$ also implies
$$ \left.\frac{d y}{d x}\right|_{x=-2}=0 \Rightarrow 12 a-4 b+c=0 \quad \ldots...(iv) $$
$y=f(x)$ cuts $y$-axis at $(0,5)$
Given, $\left.\frac{d y}{d x}\right|_{x=0}=c=3$
From (iii), (iv) and (v)
$$ \begin{aligned} &a=-\frac{1}{2}, b=-\frac{3}{4}, c=3 \\\\ &\Rightarrow f(x)=\frac{-x^{2}}{2}-\frac{3}{4} x^{2}+3 x+5 \\\\ &f^{\prime}(x)=\frac{-3}{2} x^{2}-\frac{3}{2} x+3 \\\\ &=\frac{-3}{2}(x+2)(x-1) \\\\ &f^{\prime}(x)=0 \text { at } x=-2 \text { and } x=1 \end{aligned} $$
By first derivative test $x=1$ in point of local maximum Hence local maximum value of $f(x)$ is $f(1)$
i.e., $\frac{27}{4}$
$$ \frac{d y}{d x}=3 a x^{2}+2 b x+c \quad \ldots (ii) $$
Touches $x$-axis at $P(-2,0)$
$\left.\Rightarrow y\right|_{x=-2}=0 \Rightarrow-8 a+4 b-2 c+5=0 \quad \ldots ...(iii)$
Touches $x$-axis at $P(-2,0)$ also implies
$$ \left.\frac{d y}{d x}\right|_{x=-2}=0 \Rightarrow 12 a-4 b+c=0 \quad \ldots...(iv) $$
$y=f(x)$ cuts $y$-axis at $(0,5)$
Given, $\left.\frac{d y}{d x}\right|_{x=0}=c=3$
From (iii), (iv) and (v)
$$ \begin{aligned} &a=-\frac{1}{2}, b=-\frac{3}{4}, c=3 \\\\ &\Rightarrow f(x)=\frac{-x^{2}}{2}-\frac{3}{4} x^{2}+3 x+5 \\\\ &f^{\prime}(x)=\frac{-3}{2} x^{2}-\frac{3}{2} x+3 \\\\ &=\frac{-3}{2}(x+2)(x-1) \\\\ &f^{\prime}(x)=0 \text { at } x=-2 \text { and } x=1 \end{aligned} $$
By first derivative test $x=1$ in point of local maximum Hence local maximum value of $f(x)$ is $f(1)$
i.e., $\frac{27}{4}$
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