Question
If $t =\sqrt{ x }+4$, then $\left(\frac{ dx }{ dt }\right)_{ t =4}$ is.
✓
Answer
$t =\sqrt{ x }+4$
$\Rightarrow x =( t -4)^{2}= t ^{2}-8 t +16$
$\Rightarrow \frac{ dx }{ dt }=2 t -8$
$\left.\Rightarrow \frac{ dx }{ dt }\right|_{ t =4}=2 \times 4-8=0$
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