0606 P23 - Jun 2023 - Q7 - 6 marks
7710
\(y=\frac{4x^3+2\sin 8x}{1-x}.\)
Use differentiation to find the approximate change in \(y\) as \(x\) increases from \(0.1\) to \(0.1+h\), where \(h\) is small.
Solution
Answer: The approximate change in \(y\) is \(14.3h\).
Work through the problem step by step, using exact values where possible before giving any final numerical approximation.
Let
\(y=\frac{4x^3+2\sin 8x}{1-x}.\)
Using the quotient rule,
\(\frac{dy}{dx} =\frac{(1-x)(12x^2+16\cos 8x)-(4x^3+2\sin 8x)(-1)}{(1-x)^2}.\)
Hence
\(\frac{dy}{dx} =\frac{(1-x)(12x^2+16\cos 8x)+4x^3+2\sin 8x}{(1-x)^2}.\)
At \(x=0.1\),
\(\frac{dy}{dx} =\frac{0.9(12(0.1)^2+16\cos0.8)+4(0.1)^3+2\sin0.8}{0.9^2}.\)
This gives
\(\frac{dy}{dx}\approx14.2954.\)
For a small increase \(h\) in \(x\),
\(\Delta y\approx \frac{dy}{dx}h.\)
Therefore the approximate change in \(y\) is
\(14.3h.\)