Q106 / 180 · KCET 2021 · Mathematics
The maximum slope of the curve $y=-x^{3}+3 x^{2}+2 x-27$ is
- A1
- B23
- Correct answerC5
- D$-23$
Solution
$y=-x^{3}+3 x^{2}+2 x-27$
On differentiating w.r.t. $x$, we get
$\frac{d y}{d x}=-3 x^{2}+6 x+2$
Slope of the curve $=-3 x^{2}+6 x+2$
Let $m=-3 x^{2}+6 x+2$
$\therefore \quad \frac{d m}{d x}=-6 x+6$
Put $\frac{d m}{d x}=0$ to find the critical points,
$\begin{aligned}
\Rightarrow &-6 x+6 &=0 \\
\Rightarrow \quad x &=1 \\
&\left(\frac{d^{2} m}{d x^{2}}\right)_{x=1}=-6 < 0
\end{aligned}$
Slope is maximum at $x=1$
The maximum slope,
$\begin{aligned}
m_{\max } &=-3(1)^{2}+6(1)+2 \\
&=-3+6+2=5
\end{aligned}$
Hence, the correct answer is option: 'C'.