It is given that f(x)=ln⁡(1+2x2)\mathrm{f}(x) = \ln(1 + 2x^2).

  1. (a)

    A polynomial p(x)=ax2+bx+cp(x) = ax^2 + bx + c is used to approximate f(x)\mathrm{f}(x) for 1≤x≤21 \le x \le 2. Given that p(1)=f(1)p(1) = \mathrm{f}(1), p(1.5)=f(1.5)p(1.5) = \mathrm{f}(1.5) and p(2)=f(2)p(2) = \mathrm{f}(2), find the values of the coefficients aa, bb and cc.

  2. (b)

    Using standard series from the List of Formulae (MF26), find the Maclaurin series for f(x)\mathrm{f}(x) in ascending powers of xx, up to and including the term in x6x^6.

  3. (c)

    Find the set of values of xx for the expansion in part (b) to be valid.

  4. (d)

    Use x2=16x^2 = \frac{1}{6} in your series from part (b) to show that ln⁡(43)≈m162\ln\left(\frac{4}{3}\right) \approx \frac{m}{162}, where mm is an integer to be determined.

  5. (e)

    Using your series from part (b), find the series expansion for ln⁡(1+2x21+2x+x2)\ln\left(\frac{1 + 2x^2}{1 + 2x + x^2}\right), up to and including the term in x4x^4. 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 f(x)=ln⁡(1+2x2)\mathrm{f}(x) = \ln(1 + 2x^2) and p(x)=ax2+bx+cp(x) = ax^2 + bx + c.

For x=1x = 1:
p(1)=f(1)p(1) = \mathrm{f}(1)
a(1)2+b(1)+c=ln⁡(1+2(1)2)a(1)^2 + b(1) + c = \ln(1 + 2(1)^2)
a+b+c=ln⁡3a + b + c = \ln 3 --- (1)

For x=1.5x = 1.5:
p(1.5)=f(1.5)p(1.5) = \mathrm{f}(1.5)
a(1.5)2+b(1.5)+c=ln⁡(1+2(1.5)2)a(1.5)^2 + b(1.5) + c = \ln(1 + 2(1.5)^2)
2.25a+1.5b+c=ln⁡5.52.25a + 1.5b + c = \ln 5.5 --- (2)

For x=2x = 2:
p(2)=f(2)p(2) = \mathrm{f}(2)
a(2)2+b(2)+c=ln⁡(1+2(2)2)a(2)^2 + b(2) + c = \ln(1 + 2(2)^2)
4a+2b+c=ln⁡94a + 2b + c = \ln 9 --- (3)

2. Solve the system of linear equations using a graphing calculator.

Using a graphing calculator to solve equations (1), (2) and (3) simultaneously:
a=−0.227318…a = -0.227318\dots
b=1.78056…b = 1.78056\dots
c=−0.454637…c = -0.454637\dots

Rounding to 3 significant figures:
a=−0.227a = -0.227
b=1.78b = 1.78
c=−0.455c = -0.455

Answer

a=−0.227a = -0.227, b=1.78b = 1.78, c=−0.455c = -0.455

8(b)

1. Substitute $2x^2$ into the standard series for $\ln(1+x)$.

From the List of Formulae (MF26), the standard series is:
ln⁡(1+u)=u−u22+u33−…\ln(1+u) = u - \frac{u^2}{2} + \frac{u^3}{3} - \dots

Substituting u=2x2u = 2x^2:
f(x)=(2x2)−(2x2)22+(2x2)33−…\mathrm{f}(x) = (2x^2) - \frac{(2x^2)^2}{2} + \frac{(2x^2)^3}{3} - \dots

2. Simplify the terms up to $x^6$.

=2x2−4x42+8x63−…= 2x^2 - \frac{4x^4}{2} + \frac{8x^6}{3} - \dots
=2x2−2x4+83x6= 2x^2 - 2x^4 + \frac{8}{3}x^6

Answer

2x2−2x4+83x62x^2 - 2x^4 + \frac{8}{3}x^6

8(c)

1. State the convergence condition for the standard $\ln(1+u)$ expansion

The standard series expansion for ln⁡(1+u)\ln(1+u) is valid for −1<u≤1-1 < u \le 1.
Substituting u=2x2u = 2x^2, the expansion for ln⁡(1+2x2)\ln(1+2x^2) is valid for:
−1<2x2≤1-1 < 2x^2 \le 1

2. Solve the inequality to find the range of values for $x$

Since 2x2≥02x^2 \ge 0 for all real xx, the inequality −1<2x2-1 < 2x^2 is always satisfied.
Therefore, we only need to solve:
2x2≤12x^2 \le 1
x2≤12x^2 \le \frac{1}{2}
Taking the square root of both sides gives:
−12≤x≤12-\frac{1}{\sqrt{2}} \le x \le \frac{1}{\sqrt{2}}

Answer

−12≤x≤12-\frac{1}{\sqrt{2}} \le x \le \frac{1}{\sqrt{2}}

8(d)

1. State the Maclaurin series for $\mathrm{f}(x)$ derived in part (b)

ln⁡(1+2x2)≈2x2−2x4+83x6\ln(1 + 2x^2) \approx 2x^2 - 2x^4 + \frac{8}{3}x^6

2. Substitute $x^2 = \frac{1}{6}$ into both sides of the expansion

ln⁡(1+2(16))≈2(16)−2(16)2+83(16)3\ln\left(1 + 2\left(\frac{1}{6}\right)\right) \approx 2\left(\frac{1}{6}\right) - 2\left(\frac{1}{6}\right)^2 + \frac{8}{3}\left(\frac{1}{6}\right)^3
ln⁡(1+13)≈13−2(136)+83(1216)\ln\left(1 + \frac{1}{3}\right) \approx \frac{1}{3} - 2\left(\frac{1}{36}\right) + \frac{8}{3}\left(\frac{1}{216}\right)

3. Simplify the RHS to evaluate the approximation and find $m$

ln⁡(43)≈13−118+8648\ln\left(\frac{4}{3}\right) \approx \frac{1}{3} - \frac{1}{18} + \frac{8}{648}
ln⁡(43)≈13−118+181\ln\left(\frac{4}{3}\right) \approx \frac{1}{3} - \frac{1}{18} + \frac{1}{81}
ln⁡(43)≈54162−9162+2162\ln\left(\frac{4}{3}\right) \approx \frac{54}{162} - \frac{9}{162} + \frac{2}{162}
ln⁡(43)≈47162\ln\left(\frac{4}{3}\right) \approx \frac{47}{162}
∴m=47\therefore m = 47

Answer

m=47m = 47

8(e)

1. Apply logarithm properties and factorise the denominator to split the expression.

ln⁡(1+2x21+2x+x2)=ln⁡(1+2x2)−ln⁡(1+2x+x2)\ln\left(\frac{1 + 2x^2}{1 + 2x + x^2}\right) = \ln(1 + 2x^2) - \ln(1 + 2x + x^2)
=ln⁡(1+2x2)−ln⁡((1+x)2)= \ln(1 + 2x^2) - \ln((1 + x)^2)
=ln⁡(1+2x2)−2ln⁡(1+x)= \ln(1 + 2x^2) - 2\ln(1 + x)

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 ln⁡(1+2x2)≈2x2−2x4\ln(1 + 2x^2) \approx 2x^2 - 2x^4.
Using the standard series for ln⁡(1+x)\ln(1+x) up to the term in x4x^4:
2ln⁡(1+x)≈2(x−x22+x33−x44)=2x−x2+23x3−12x42\ln(1 + x) \approx 2\left(x - \frac{x^2}{2} + \frac{x^3}{3} - \frac{x^4}{4}\right) = 2x - x^2 + \frac{2}{3}x^3 - \frac{1}{2}x^4

Subtracting the two series:
ln⁡(1+2x21+2x+x2)≈(2x2−2x4)−(2x−x2+23x3−12x4)\ln\left(\frac{1 + 2x^2}{1 + 2x + x^2}\right) \approx (2x^2 - 2x^4) - \left(2x - x^2 + \frac{2}{3}x^3 - \frac{1}{2}x^4\right)
=−2x+3x2−23x3−32x4= -2x + 3x^2 - \frac{2}{3}x^3 - \frac{3}{2}x^4

Answer

−2x+3x2−23x3−32x4-2x + 3x^2 - \frac{2}{3}x^3 - \frac{3}{2}x^4

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