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 and differentiate implicitly with respect to $x$.
Differentiating implicitly with respect to :
2. Express the derivative in terms of $y$ and simplify to obtain the required result.
From , we can express as .
Substituting this into the differentiated equation:
Dividing both sides by :
(Shown)
9(b)
1. Evaluate $y$ and the first derivative $\frac{dy}{dx}$ at $x=0$.
At ,
Using the result from part (a),
2. Differentiate implicitly to find the second derivative $\frac{d^2y}{dx^2}$ and evaluate it at $x=0$.
Substituting and ,
3. Apply the Maclaurin series expansion formula up to the $x^2$ term.
The Maclaurin series expansion is:
Substituting the evaluated values:
9(c)
1. Establish the result from part (b) that needs to be checked.
From part (a), .
At , .
.
Differentiating implicitly, .
.
The Maclaurin series from part (b) is:
.
2. Expand $e^{-2x}$ up to the term in $x^2$ using the standard series.
Using the standard series ,
3. Substitute the expansion into the logarithmic function.
4. Expand $\ln(1+u)$ using the standard series, where $u = 2x - 2x^2$.
Using the standard series , substitute :
5. Expand and simplify up to the $x^2$ term, and conclude the check.
Expanding the squared term up to gives .
This exactly matches the Maclaurin series derived in part (b), confirming its correctness.
(which matches the series in part (b))
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