For the curve $y=5 x-2 x^3$, if $x$ increases at the rate of 2 units/sec, the rate of change in the slope of…

For the curve $y=5 x-2 x^3$, if $x$ increases at the rate of 2 units/sec, the rate of change in the slope of the curve at $x=3$ is ......../sec
  1. $72$
  2. $27$
  3. $-72$
  4. $-27$

Solution

$y=5 x-2 x^3$ $\frac{d y}{d x}=5-6 x^2$ $\therefore$ Slope of curve is $5-6 x^2$. $\therefore \quad m=5-6 x^2 \Rightarrow \frac{d m}{d t}=(-12 x) \frac{d x}{d t}$ $\left(\frac{d m}{d t}\right)_{x=3}=-12(3) \times 2=-72$ $\left[\because \frac{d x}{d t}=2\right]$ $\Rightarrow$ Rate of change of slope at $x=3$ is -72

Asked in: AP EAMCET 2021 (24 Aug Shift 2)

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