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CBSE Class 12 Math 2012 Solved Paper

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Question : 22 of 29
Marks: +1, -0
Find the particular solution of the following differential equation:
(x + 1) dydx\frac{dy}{dx} = 2e−y2e^{-y} - 1 ; y = 0 when x = 0
Solution:  
(x + 1) dydx\frac{dy}{dx} = 2e−y2e^{-y} - 1
⇒ dy2e−y−1\frac{dy}{2e^{-y}-1} = dxx+1\frac{dx}{x+1}
⇒ ey2−ey\frac{e^y}{2-e^y} = dxx+1\frac{dx}{x+1}
Integrating both sides, we get:
∫ eydy2−ey\frac{e^y dy}{2-e^y} = log |x + 1| + log C ... (1)
Let 2 - eye^y = t
∴ ddy(2−ey)=\frac{d}{dy}(2-e^y) ={dt}/{dy}$⇒−\Rightarrow -e^y=={dt}/{dy}⇒\Rightarrowe^ydy=−dt Substituting this value in equation 1 , we get: ∫−dy = - dt \text{ Substituting this value in equation 1 , we get: } \int -{dt}/t=log⁡∣x+1∣+log⁡C⇒−log⁡∣t∣=log⁡∣C(x+1)∣⇒−log⁡∣2−= \log |x + 1| + \log C \Rightarrow - \log |t| = \log |C (x + 1)| \Rightarrow - \log |2 -e^y,∣=log⁡∣C(x+1)∣⇒,| = \log |C (x + 1)| \Rightarrow1/{2-e^y},=C(x+1)⇒2−, = C (x + 1) \Rightarrow 2 -e^y,=, =1/{C(x+1)},…(2) Now, at x=0 and y=0, equation 2 becomes: ⇒2−1=, \ldots (2) \text{ Now, at } x = 0 \text{ and } y = 0, \text{ equation 2 becomes: } \Rightarrow 2 - 1 =1/C,,e^y⇒C=1 Substituting C=1 in equation 2, we get: 2−\Rightarrow C = 1 \text{ Substituting } C = 1 \text{ in equation 2, we get: } 2 -1/{x+1},=, =e^y,⇒, \Rightarrow1/{x+1},=2−, = 2 -e^y,⇒, \Rightarrow{2x+2-1}/{x+1},=, =e^y,⇒, \Rightarrow{2x+1}/{x+1},=, =|{2x+1}/{x+1}|$ , (x ≠ - 1)
This is the required particular solution of the given differential equation.
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