0606 P22 - Mar 2019 - Q7 - 6 marks
8247
(i) Given that \(y=x\sqrt{x^2+1}\), show that \(\dfrac{dy}{dx}=\dfrac{ax^2+b}{(x^2+1)^p}\), where \(a\), \(b\) and \(p\) are positive constants.
(ii) Explain why the graph of \(y=x\sqrt{x^2+1}\) has no stationary points.
Solution
Answer: \(\dfrac{dy}{dx}=\dfrac{2x^2+1}{(x^2+1)^{1/2}}\); no stationary points because \(\dfrac{dy}{dx}\gt 0\) for all real \(x\).
Write
\(y=x(x^2+1)^{1/2}.\)
Using the product rule,
\(\frac{dy}{dx}=(x^2+1)^{1/2} +x\cdot\frac12(x^2+1)^{-1/2}\cdot2x.\)
So
\(\frac{dy}{dx}=(x^2+1)^{1/2}+x^2(x^2+1)^{-1/2}.\)
Put over a common denominator:
\(\frac{dy}{dx}=\frac{x^2+1+x^2}{(x^2+1)^{1/2}} =\frac{2x^2+1}{(x^2+1)^{1/2}}.\)
Hence \(a=2\), \(b=1\) and \(p=\frac12\).
The numerator \(2x^2+1\) is always positive, and the denominator \((x^2+1)^{1/2}\) is also always positive. Therefore
\(\frac{dy}{dx}\gt 0\)
for all real \(x\), so the graph has no stationary points.