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Solve the following differential equation: x^2 . dy/dx = x^2 + xy + y^2 - Mathematics and Statistics

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Question

Solve the following differential equation:

`x^2.  dy/dx = x^2 + xy + y^2`

Sum
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Solution

`x^2.  dy/dx = x^2 + xy + y^2`

∴ `dy/dx = (x^2 + xy + y^2)/x^2`    ...(1)

Put y = vx

∴ `dy/dx = v + x (dv)/dx`

∴ (1) becomes, `v + x (dv)/dx = (x^2 + x * vx + v^2x^2)/x^2`

∴ `v + x (dv)/dx = 1 + v + v^2`

∴ `x (dv)/dx = 1 + v^2`

∴ `1/(1 + v^2) dv = 1/x dx`

Integrating, we get

`int 1/(1 + v^2) dv = int 1/x dx + c`

∴ tan–1 v = log |x| + c

∴  `tan^-1 (y/x) = log |x| + c`

This is the general solution.

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Chapter 6: Differential Equations - Exercise 6.4 [Page 203]

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