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CBSE Class 12 Math 2013 Solved Paper

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Question : 26 of 29
Marks: +1, -0
Show that the differential equation 2yex/y2ye^{x/y} dx + (y - 2x ex/ye^{x/y}) dy is homogeneous. Find the particular solution of this differential equation, given that x = 0 when y = 1.= 0
Solution:  
2yex/y2ye^{x/y} dx + (y - 2x ex/ye^{x/y}) dy = 0
dxdy\frac{dx}{dy} = 2xex/y−y2yex/y\frac{2xe^{x/y}-y}{2ye^{x/y}} ... (1)
Let f (x , y) = 2xex/y−y2yex/y\frac{2xe^{x/y}-y}{2ye^{x/y}}
Then, (λx , λy) = λ(2xex/y−y)λ(2yex/y)\frac{\lambda(2xe^{x/y}-y)}{\lambda(2ye^{x/y})} = λ0\lambda^0 [F (x,y)]
Thus, F(x, y) is a homogeneous function of degree zero. Therefore, the given differential equation is a homogeneous differential equation.
Let x = vy
Differentiating w.r.t. y, we get
dxdy\frac{dx}{dy} = v + y dvdy\frac{dv}{dy}
Substituting the value of x and dxdy\frac{dx}{dy} in equation (1), we get
v + y dvdy\frac{dv}{dy} = 2vyev−y2yev\frac{2vye^v-y}{2ye^v} = 2vev−12ev\frac{2ve^v-1}{2e^v}
or y dvdy\frac{dv}{dy} = 2vev−12ev\frac{2ve^v-1}{2e^v} - v
or y dvdy\frac{dv}{dy} = - 12ev\frac{1}{2e^v}
or 2ev2e^v dv = - dyy\frac{dy}{y}
or ∫ 2ev2e^v . dv = - ∫ dyy\frac{dy}{y}
or 2ev2e^v = - log |y| + C
Substituting the value of v, we get
2eπ/y2e^{\pi/y} + log |y| = C ... (2)
Substituting x = 0 and y = 1 in equation (2), we get
2e02e^0 + log |1| = C ⇒ C = 2
Substituting the value of C in equation (2), we get
2ex/y2e^{x/y} + log |y| = 2, which is the particular solution of the given differential equation.
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