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Reduce the following differential equation to the variable separable form and hence solve: x + ydydxxyx + ydydx=sec(x2+y2)

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Question

Reduce the following differential equation to the variable separable form and hence solve:

`"x + y""dy"/"dx" = sec("x"^2 + "y"^2)`

Sum
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Solution

`"x + y""dy"/"dx" = sec("x"^2 + "y"^2)`   ....(1)

Put x2 + y2 = u

∴ 2x + 2y`"dy"/"dx" = "du"/"dx"`

∴ x + y`"dy"/"dx" = 1/2 * "du"/"dx"`

∴ (1) becomes, `1/2 * "du"/"dx" = sec"u"`

∴ `1/(sec "u") = 2 * "dx"`

Integrating both sides, we get

∫ cos u du = 2 ∫ dx

∴ sin u = 2x + c

∴ sin (x2 + y2) = 2x + c

This is the general solution.

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Chapter 6: Differential Equations - Exercise 6.3 [Page 201]

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