It is given that .
- (a)
A polynomial is used to approximate for . Given that , and , find the values of the coefficients , and .
- (b)
Using standard series from the List of Formulae (MF26), find the Maclaurin series for in ascending powers of , up to and including the term in .
- (c)
Find the set of values of for the expansion in part (b) to be valid.
- (d)
Use in your series from part (b) to show that , where is an integer to be determined.
- (e)
Using your series from part (b), find the series expansion for , up to and including the term in . Give the coefficients in exact form.
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Worked solution
8(a)
1. Substitute the given values of $x$ to form a system of linear equations in $a$, $b$, and $c$.
Given and .
For :
--- (1)
For :
--- (2)
For :
--- (3)
2. Solve the system of linear equations using a graphing calculator.
Using a graphing calculator to solve equations (1), (2) and (3) simultaneously:
Rounding to 3 significant figures:
, ,
8(b)
1. Substitute $2x^2$ into the standard series for $\ln(1+x)$.
From the List of Formulae (MF26), the standard series is:
Substituting :
2. Simplify the terms up to $x^6$.
8(c)
1. State the convergence condition for the standard $\ln(1+u)$ expansion
The standard series expansion for is valid for .
Substituting , the expansion for is valid for:
2. Solve the inequality to find the range of values for $x$
Since for all real , the inequality is always satisfied.
Therefore, we only need to solve:
Taking the square root of both sides gives:
8(d)
1. State the Maclaurin series for $\mathrm{f}(x)$ derived in part (b)
2. Substitute $x^2 = \frac{1}{6}$ into both sides of the expansion
3. Simplify the RHS to evaluate the approximation and find $m$
8(e)
1. Apply logarithm properties and factorise the denominator to split the expression.
2. Use the standard series expansion for $\ln(1+x)$ and subtract it from the expansion of $\ln(1+2x^2)$ found in part (b).
From part (b), we have .
Using the standard series for up to the term in :
Subtracting the two series:
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