It is given that y3+y2+2y=x2−3xy^3 + y^2 + 2y = x^2 - 3x, find the Maclaurin series for yy up to and including the term in x2x^2. Hence find the Maclaurin series for 12+y\frac{1}{2 + y} up to and including the term in x2x^2.

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Worked solution

7(i)

1. Find the value of $y$ when $x = 0$.

Substituting x=0x = 0 into the given equation:
y3+y2+2y=(0)2−3(0)y^3 + y^2 + 2y = (0)^2 - 3(0)
y(y2+y+2)=0y(y^2 + y + 2) = 0
Since y2+y+2=(y+12)2+74>0y^2 + y + 2 = (y + \frac{1}{2})^2 + \frac{7}{4} > 0 for all real yy, the quadratic factor has no real roots. Therefore, the only real root is:
y=0y = 0

2. Differentiate the equation implicitly to find the value of the first derivative at $x = 0$.

Differentiating implicitly with respect to xx:
3y2dydx+2ydydx+2dydx=2x−33y^2 \frac{dy}{dx} + 2y \frac{dy}{dx} + 2 \frac{dy}{dx} = 2x - 3
(3y2+2y+2)dydx=2x−3(3y^2 + 2y + 2)\frac{dy}{dx} = 2x - 3
Substitute x=0x = 0 and y=0y = 0:
(0+0+2)dydx=2(0)−3(0 + 0 + 2)\frac{dy}{dx} = 2(0) - 3
2dydx=−3  ⟹  dydx=−322\frac{dy}{dx} = -3 \implies \frac{dy}{dx} = -\frac{3}{2}

3. Differentiate implicitly a second time to find the value of the second derivative at $x = 0$.

Differentiating (3y2+2y+2)dydx=2x−3(3y^2 + 2y + 2)\frac{dy}{dx} = 2x - 3 with respect to xx using the product rule:
(6ydydx+2dydx)dydx+(3y2+2y+2)d2ydx2=2\left(6y \frac{dy}{dx} + 2 \frac{dy}{dx}\right)\frac{dy}{dx} + (3y^2 + 2y + 2)\frac{d^2y}{dx^2} = 2
Substitute x=0x = 0, y=0y = 0, and dydx=−32\frac{dy}{dx} = -\frac{3}{2}:
(0+2(−32))(−32)+(0+0+2)d2ydx2=2\left( 0 + 2\left(-\frac{3}{2}\right) \right)\left(-\frac{3}{2}\right) + (0 + 0 + 2)\frac{d^2y}{dx^2} = 2
2(−32)2+2d2ydx2=22\left(-\frac{3}{2}\right)^2 + 2\frac{d^2y}{dx^2} = 2
92+2d2ydx2=2\frac{9}{2} + 2\frac{d^2y}{dx^2} = 2
2d2ydx2=2−92=−522\frac{d^2y}{dx^2} = 2 - \frac{9}{2} = -\frac{5}{2}
d2ydx2=−54\frac{d^2y}{dx^2} = -\frac{5}{4}

4. Construct the Maclaurin series for $y$ up to the term in $x^2$.

The Maclaurin series expansion is given by:
y=y(0)+y′(0)x+y′′(0)2!x2+…y = y(0) + y'(0)x + \frac{y''(0)}{2!}x^2 + \dots
Substituting the evaluated derivatives:
y=0+(−32)x+−542x2y = 0 + \left(-\frac{3}{2}\right)x + \frac{-\frac{5}{4}}{2}x^2
y=−32x−58x2y = -\frac{3}{2}x - \frac{5}{8}x^2

Answer

y=−32x−58x2y = -\frac{3}{2}x - \frac{5}{8}x^2

7(ii)

1. Derive the Maclaurin series for $y$ up to the term in $x^2$ (from part (i)).

Substitute x=0x = 0 into y3+y2+2y=x2−3xy^3 + y^2 + 2y = x^2 - 3x:
y(y2+y+2)=0  ⟹  y=0y(y^2 + y + 2) = 0 \implies y = 0.

Differentiate implicitly with respect to xx:
3y2dydx+2ydydx+2dydx=2x−33y^2 \frac{dy}{dx} + 2y \frac{dy}{dx} + 2 \frac{dy}{dx} = 2x - 3.
Substitute x=0,y=0  ⟹  2dydx=−3  ⟹  dydx=−32x=0, y=0 \implies 2 \frac{dy}{dx} = -3 \implies \frac{dy}{dx} = -\frac{3}{2}.

Differentiate again:
6y(dydx)2+3y2d2ydx2+2(dydx)2+2yd2ydx2+2d2ydx2=26y \left(\frac{dy}{dx}\right)^2 + 3y^2 \frac{d^2y}{dx^2} + 2\left(\frac{dy}{dx}\right)^2 + 2y \frac{d^2y}{dx^2} + 2 \frac{d^2y}{dx^2} = 2.
Substitute x=0,y=0,dydx=−32x=0, y=0, \frac{dy}{dx}=-\frac{3}{2}:
0+0+2(−32)2+0+2d2ydx2=20 + 0 + 2\left(-\frac{3}{2}\right)^2 + 0 + 2 \frac{d^2y}{dx^2} = 2
92+2d2ydx2=2  ⟹  2d2ydx2=−52  ⟹  d2ydx2=−54\frac{9}{2} + 2 \frac{d^2y}{dx^2} = 2 \implies 2 \frac{d^2y}{dx^2} = -\frac{5}{2} \implies \frac{d^2y}{dx^2} = -\frac{5}{4}.

The Maclaurin series for yy is:
y≈y(0)+y′(0)x+y′′(0)2!x2=−32x−58x2y \approx y(0) + y'(0)x + \frac{y''(0)}{2!}x^2 = -\frac{3}{2}x - \frac{5}{8}x^2.

2. Expand $\frac{1}{2 + y}$ using the standard binomial series up to the term in $y^2$.

Rewrite the expression to use the standard series (1+x)n(1+x)^n:
12+y=(2+y)−1=2−1(1+y2)−1\frac{1}{2 + y} = (2 + y)^{-1} = 2^{-1} \left(1 + \frac{y}{2}\right)^{-1}

Expand using the binomial theorem:
=12[1+(−1)(y2)+(−1)(−2)2!(y2)2+… ]= \frac{1}{2} \left[ 1 + (-1)\left(\frac{y}{2}\right) + \frac{(-1)(-2)}{2!} \left(\frac{y}{2}\right)^2 + \dots \right]
=12(1−y2+y24−… )= \frac{1}{2} \left( 1 - \frac{y}{2} + \frac{y^2}{4} - \dots \right)
=12−14y+18y2−…= \frac{1}{2} - \frac{1}{4}y + \frac{1}{8}y^2 - \dots

3. Substitute the Maclaurin series for $y$ into the expansion and simplify up to $x^2$.

Substitute y=−32x−58x2y = -\frac{3}{2}x - \frac{5}{8}x^2 into the expanded series:
12+y≈12−14(−32x−58x2)+18(−32x−58x2)2\frac{1}{2+y} \approx \frac{1}{2} - \frac{1}{4}\left(-\frac{3}{2}x - \frac{5}{8}x^2\right) + \frac{1}{8}\left(-\frac{3}{2}x - \frac{5}{8}x^2\right)^2

Expand and ignore terms of degree higher than 2:
≈12+38x+532x2+18(−32x)2\approx \frac{1}{2} + \frac{3}{8}x + \frac{5}{32}x^2 + \frac{1}{8}\left( -\frac{3}{2}x \right)^2
=12+38x+532x2+18(94x2)= \frac{1}{2} + \frac{3}{8}x + \frac{5}{32}x^2 + \frac{1}{8}\left( \frac{9}{4}x^2 \right)
=12+38x+532x2+932x2= \frac{1}{2} + \frac{3}{8}x + \frac{5}{32}x^2 + \frac{9}{32}x^2
=12+38x+1432x2= \frac{1}{2} + \frac{3}{8}x + \frac{14}{32}x^2
=12+38x+716x2= \frac{1}{2} + \frac{3}{8}x + \frac{7}{16}x^2

Answer

\frac{1}{2} + \frac{3}{8}x + \frac{7}{16}x^2

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