It is given that .
- (a)
Show that .
- (b)
Hence find the Maclaurin series for , up to and including the term in .
- (c)
Using standard series from the List of Formulae (MF26), expand as far as the term in , and use this expansion as a check on the correctness of the series found in part (b).
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Worked solution
9(a)
1. Rewrite the equation in exponential form to prepare for implicit differentiation without relying on fractions.
2. Differentiate both sides implicitly with respect to $x$.
3. Substitute $\text{e}^{-2x}$ in terms of $y$ and rearrange to obtain the required expression.
From the first step, we know that . Substituting this in:
Divide both sides by :
(Shown)
9(b)
1. Evaluate $y$ and $\frac{\mathrm{d}y}{\mathrm{d}x}$ at $x = 0$.
Substitute into the original equation:
Substitute into the result from part (a):
2. Differentiate $\frac{\mathrm{d}y}{\mathrm{d}x}$ implicitly to find the second derivative.
3. Evaluate the second derivative at $x = 0$ and construct the Maclaurin series.
At :
The Maclaurin series is given by:
9(c)
1. Expand $\text{e}^{-2x}$ up to the term in $x^2$ using the standard series.
2. Substitute the expansion of $\text{e}^{-2x}$ into $2 - \text{e}^{-2x}$.
3. Expand $\ln(1 + u)$ where $u = 2x - 2x^2$, up to the term in $x^2$.
Using the standard series :
Discarding terms with powers higher than :
4. Compare the obtained expansion with the series found in part (b) to verify correctness.
In part (b), evaluating , , and at yields , , and respectively, giving the Maclaurin series .
Since the expansion using standard series is also , it confirms the correctness of the series found in part (b).
, which matches the series from part (b).
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