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The substitution y=vxy=vx transforms the differential equation xdy−ydx=x2+y2dxxdy-ydx=\sqrt{x^2+y^2}dx into

Solution

✅ Correct Option: 1

With y=vxy = vx, dy=v dx+x dvdy = v\,dx + x\,dv.

Then x(v dx+x dv)−vx dx=x2dvx(v\,dx + x\,dv) - vx\,dx = x^2 dv, while the right side becomes x1+v2 dxx\sqrt{1+v^2}\,dx.

So x2dv=x1+v2 dxx^2 dv = x\sqrt{1+v^2}\,dx, giving dv1+v2=dxx\dfrac{dv}{\sqrt{1+v^2}} = \dfrac{dx}{x}.

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