It is given that y=ln⁡(2−e−2x).y = \ln\left(2 - e^{-2x}\right).

  1. (a)

    Show that dydx=4e−y−2\dfrac{dy}{dx} = 4e^{-y} - 2.

  2. (b)

    Hence find the Maclaurin series for yy, up to and including the term in x2x^2.

  3. (c)

    Using standard series from the List of Formulae (MF26), expand ln⁡(2−e−2x)\ln\left(2 - e^{-2x}\right) as far as the term in x2x^2, 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 $x$.

y=ln⁡(2−e−2x)  ⟹  ey=2−e−2xy = \ln\left(2 - e^{-2x}\right) \implies e^y = 2 - e^{-2x}

Differentiating implicitly with respect to xx:
eydydx=2e−2xe^y \frac{dy}{dx} = 2e^{-2x}

2. Express the derivative in terms of $y$ and simplify to obtain the required result.

From ey=2−e−2xe^y = 2 - e^{-2x}, we can express e−2xe^{-2x} as 2−ey2 - e^y.
Substituting this into the differentiated equation:
eydydx=2(2−ey)e^y \frac{dy}{dx} = 2(2 - e^y)
eydydx=4−2eye^y \frac{dy}{dx} = 4 - 2e^y
Dividing both sides by eye^y:
dydx=4−2eyey\frac{dy}{dx} = \frac{4 - 2e^y}{e^y}
dydx=4e−y−2 (Shown)\frac{dy}{dx} = 4e^{-y} - 2 \text{ (Shown)}

Answer

dydx=4e−y−2\frac{dy}{dx} = 4e^{-y} - 2 (Shown)

9(b)

1. Evaluate $y$ and the first derivative $\frac{dy}{dx}$ at $x=0$.

At x=0x = 0,
y(0)=ln⁡(2−e0)=ln⁡(1)=0y(0) = \ln(2 - e^0) = \ln(1) = 0

Using the result from part (a),
dydx∣x=0=4e0−2=2\frac{dy}{dx}\bigg|_{x=0} = 4e^0 - 2 = 2

2. Differentiate implicitly to find the second derivative $\frac{d^2y}{dx^2}$ and evaluate it at $x=0$.

d2ydx2=ddx(4e−y−2)\frac{d^2y}{dx^2} = \frac{d}{dx}\left(4e^{-y} - 2\right)
d2ydx2=−4e−ydydx\frac{d^2y}{dx^2} = -4e^{-y}\frac{dy}{dx}

Substituting y=0y=0 and dydx=2\frac{dy}{dx}=2,
d2ydx2∣x=0=−4(e0)(2)=−8\frac{d^2y}{dx^2}\bigg|_{x=0} = -4(e^0)(2) = -8

3. Apply the Maclaurin series expansion formula up to the $x^2$ term.

The Maclaurin series expansion is:
y=y(0)+y′(0)x+y′′(0)2!x2+…y = y(0) + y'(0)x + \frac{y''(0)}{2!}x^2 + \ldots

Substituting the evaluated values:
y=0+2x+−82x2y = 0 + 2x + \frac{-8}{2}x^2
y=2x−4x2y = 2x - 4x^2

Answer

2x−4x22x - 4x^2

9(c)

1. Establish the result from part (b) that needs to be checked.

From part (a), dydx=4e−y−2\frac{dy}{dx} = 4e^{-y} - 2.
At x=0x = 0, y=ln⁡(2−e0)=0y = \ln(2 - e^0) = 0.
dydx∣x=0=4(1)−2=2\frac{dy}{dx} \Big|_{x=0} = 4(1) - 2 = 2.
Differentiating implicitly, d2ydx2=−4e−ydydx\frac{d^2y}{dx^2} = -4e^{-y}\frac{dy}{dx}.
d2ydx2∣x=0=−4(1)(2)=−8\frac{d^2y}{dx^2} \Big|_{x=0} = -4(1)(2) = -8.
The Maclaurin series from part (b) is:
y=y(0)+y′(0)x+y′′(0)2!x2=2x−4x2y = y(0) + y'(0)x + \frac{y''(0)}{2!}x^2 = 2x - 4x^2.

2. Expand $e^{-2x}$ up to the term in $x^2$ using the standard series.

Using the standard series ex=1+x+x22!+…e^x = 1 + x + \frac{x^2}{2!} + \dots,
e−2x=1+(−2x)+(−2x)22!+…e^{-2x} = 1 + (-2x) + \frac{(-2x)^2}{2!} + \dots
e−2x≈1−2x+2x2e^{-2x} \approx 1 - 2x + 2x^2

3. Substitute the expansion into the logarithmic function.

y=ln⁡(2−e−2x)y = \ln(2 - e^{-2x})
y≈ln⁡(2−(1−2x+2x2))y \approx \ln(2 - (1 - 2x + 2x^2))
y≈ln⁡(1+2x−2x2)y \approx \ln(1 + 2x - 2x^2)

4. Expand $\ln(1+u)$ using the standard series, where $u = 2x - 2x^2$.

Using the standard series ln⁡(1+u)=u−u22+…\ln(1+u) = u - \frac{u^2}{2} + \dots, substitute u=2x−2x2u = 2x - 2x^2:
y≈(2x−2x2)−12(2x−2x2)2y \approx (2x - 2x^2) - \frac{1}{2}(2x - 2x^2)^2

5. Expand and simplify up to the $x^2$ term, and conclude the check.

Expanding the squared term up to x2x^2 gives (2x)2=4x2(2x)^2 = 4x^2.
y≈2x−2x2−12(4x2)y \approx 2x - 2x^2 - \frac{1}{2}(4x^2)
y≈2x−2x2−2x2y \approx 2x - 2x^2 - 2x^2
y≈2x−4x2y \approx 2x - 4x^2

This exactly matches the Maclaurin series derived in part (b), confirming its correctness.

Answer

y≈2x−4x2y \approx 2x - 4x^2 (which matches the series in part (b))

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