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Exercise 6.2 · Q13

Q.Obtain the differential equation by eliminating the arbitrary constants: Ax2+By2=1Ax^2+By^2=1

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Ax2+By2=1Ax^2+By^2=1 has two arbitrary constants, so differentiate twice. First: 2Ax+2Bydydx=02Ax+2By\dfrac{dy}{dx}=0, i.e. Ax+Bydydx=0Ax+By\dfrac{dy}{dx}=0. Differentiate again: A+B[(dydx)2+yd2ydx2]=0A+B\left[\left(\dfrac{dy}{dx}\right)^2+y\dfrac{d^2y}{dx^2}\right]=0, so A=−B[(dydx)2+yd2ydx2]A=-B\left[\left(\dfrac{dy}{dx}\right)^2+y\dfrac{d^2y}{dx^2}\right]. From the first equation, A=−By dy/dxxA=-\dfrac{By\,dy/dx}{x}. Equating the two expressions for AA and cancelling BB: y dy/dxx=(dydx)2+yd2ydx2\dfrac{y\,dy/dx}{x}=\left(\dfrac{dy}{dx}\right)^2+y\dfrac{d^2y}{dx^2}, i.e. xyd2ydx2+x(dydx)2−ydydx=0xy\dfrac{d^2y}{dx^2}+x\left(\dfrac{dy}{dx}\right)^2-y\dfrac{dy}{dx}=0.

✓Final answer

xyd2ydx2+x(dydx)2−ydydx=0xy\dfrac{d^2y}{dx^2}+x\left(\dfrac{dy}{dx}\right)^2-y\dfrac{dy}{dx}=0

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