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For x>y>0x > y > 0, if x5y6=(x+y)11x^5 y^6 = (x + y)^{11}, then d2ydx2\frac{d^2y}{dx^2} is

Solution

Correct Option: 4

The equation x5y6=(x+y)11x^5 y^6 = (x+y)^{11} is homogeneous since both sides have degree 1111.

Substituting y=vxy = vx:

x5(vx)6=(x+vx)11x^5 (vx)^6 = (x + vx)^{11}

v6x11=(1+v)11x11v^6 \cdot x^{11} = (1 + v)^{11} \cdot x^{11}

Cancelling x11x^{11} (valid since x>0x > 0):

v6=(1+v)11v^6 = (1 + v)^{11}


This equation has no xx, so vv is a constant.

Since y=vxy = vx where vv is constant:

dydx=v\dfrac{dy}{dx} = v

d2ydx2=ddx(v)=0\dfrac{d^2y}{dx^2} = \dfrac{d}{dx}(v) = 0

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