Porisma

Worked solution

Reduce order and respect a singular boundary

(i) Use z=(2x−1)dy/dxz=(2x-1)dy/dx to express (2x−1)d2y/dx2=2(1−dy/dx)(2x-1)d^2y/dx^2=2(1-dy/dx) as an equation in z,xz,x. marks

(ii) Solve with y=2y=2 and dy/dx=2dy/dx=2 at x=0x=0. marks

(iii) Describe the gradient as (a) x→±∞x\to\pm\infty mark and (b) x→1/2x\to1/2. mark

(iv) Sketch the solution from (ii). marks

[2013/IJC/II/4(modified)]

CJC · Review problems · Question 3

Answer

z′=2z^{\prime}=2; y=x−12ln⁡∣2x−1∣+2y=x-\tfrac12\ln|2x-1|+2, with connected initial-value domain x<1/2x<1/2. Gradient limits of the formal branches are 11 at infinity and +∞/−∞+\infty/-\infty on the left/right of 1/21/2.

(i) Recognise the product derivative

z′=(2x−1)y′′+2y′z^{\prime}=(2x-1)y^{\prime\prime}+2y^{\prime}. The given equation rearranges to (2x−1)y′′+2y′=2(2x-1)y^{\prime\prime}+2y^{\prime}=2, so dz/dx=2dz/dx=2.

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(ii) Apply the derivative condition before integrating y

z=2x+Cz=2x+C, so (2x−1)y′=2x+C(2x-1)y^{\prime}=2x+C. At x=0x=0, y′=2y^{\prime}=2 gives C=−2C=-2, hence y′=(2x−2)/(2x−1)=1−1/(2x−1)y^{\prime}=(2x-2)/(2x-1)=1-1/(2x-1). Integration gives y=x−12ln⁡∣2x−1∣+By=x-\tfrac12\ln|2x-1|+B; y(0)=2y(0)=2 gives B=2B=2.

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(ii) Identify the connected initial-value domain

The initial point lies on x<1/2x<1/2. Its maximal connected solution is y=x−12ln⁡(1−2x)+2y=x-\tfrac12\ln(1-2x)+2 on (−∞,1/2)(-\infty,1/2). The formula with the absolute value can also be drawn for x>1/2x>1/2 using the same constant, as the source does, but this is a separate formal branch. The divergent curve cannot be continued through x=1/2x=1/2, and the initial conditions do not fix an independent integration constant on the other side.

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(iii) Compute the gradient limits from the correct derivative

For the displayed two-branch formula, y′→1y^{\prime}\to1 as x→±∞x\to\pm\infty; y′→+∞y^{\prime}\to+\infty as x→(1/2)−x\to(1/2)^- and y′→−∞y^{\prime}\to-\infty as x→(1/2)+x\to(1/2)^+. Only the negative-infinity and left-hand singular limits belong to the connected initial-value solution. A source line in (iii) prints 1−1/[2(2x−1)]1-1/[2(2x-1)], inserting an extra factor of two; the earlier derivation and the derivative of the stated solution give 1−1/(2x−1)1-1/(2x-1). The reported limit signs remain correct.

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(iv) Sketch and distinguish the two branches

The initial-value branch passes through (0,2)(0,2) and increases to +∞+\infty at the vertical asymptote x=1/2x=1/2. The independently displayed right branch decreases from +∞+\infty, reaches (1,3)(1,3), and then increases. The sketch uses a solid initial-value branch and a dashed formal right branch. A limiting gradient of 11 does not establish an oblique line asymptote: the logarithmic term continues to grow in magnitude.

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Compare your sketch

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Solid blue initial-value branch y=x−(1/2)ln|2x−1|+2 on x<1/2 passes through (0,2) and diverges at x=1/2. A dashed brown formal right branch has minimum (1,3); it is not a continuation through the singularity.
Solid blue initial-value branch y=x−(1/2)ln|2x−1|+2 on x<1/2 passes through (0,2) and diverges at x=1/2. A dashed brown formal right branch has minimum (1,3); it is not a continuation through the singularity.
Solid blue initial-value branch y=x−(1/2)ln|2x−1|+2 on x<1/2 passes through (0,2) and diverges at x=1/2. A dashed brown formal right branch has minimum (1,3); it is not a continuation through the singularity.

Common mistakes

The branch on the other side of a vertical singularity is not determined by initial values at x=0.

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