CBSE 2025 · Region 4 · Set 1 · Q25 · 2 marks
For the curve $\displaystyle y=5 x-2 x^{3}$, if $\displaystyle x$ increases at the rate of $\displaystyle 2$ units $\displaystyle / \mathrm{s}$, then how fast is the slope of the curve changing when $\displaystyle x=2$ ?
Marking-scheme solution
\[y=5 x-2 x^{3}
\]
Given $\displaystyle \frac{d x}{d t}=2$ units $\displaystyle / \mathrm{s}$
slope of the curve $\displaystyle =\frac{d y}{d x}=5-6 x^{2}=m$
\[\frac{d m}{d t}=-12 x \frac{d x}{d t}=-12 x(2)=-24 x
\]
at $\displaystyle x=2, \frac{d m}{d t}=-24(2)=-48$
Hence,slope of curveis decreasing at the rate of $\displaystyle 48$
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CBSE Class 12 Mathematics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.