JEE Main 2021 · 31 August · Evening Shift · Mathematics · Differential Equations
A. (1,2)\left(1, 2\right)(1,2)
Given differential equation is
dydx=2x⋅y+2y⋅2x2x+2x+yloge2\dfrac{dy}{dx} = \dfrac{{2}^{x} \cdot y + {2}^{y} \cdot {2}^{x}}{{2}^{x} + {2}^{x + y}\log_{e}2}dxdy=2x+2x+yloge22x⋅y+2y⋅2x
⇒dydx=2x(y+2y)2x(1+2yloge2)⇒ \dfrac{dy}{dx} = \dfrac{{2}^{x}\left(y + {2}^{y}\right)}{{2}^{x}\left(1 + {2}^{y}\log_{e}2\right)}⇒dxdy=2x(1+2yloge2)2x(y+2y)
⇒∫1+2yloge2y+2ydy=∫dx⇒ ∫ \dfrac{1 + {2}^{y}\log_{e}2}{y + {2}^{y}}dy = ∫ dx⇒∫y+2y1+2yloge2dy=∫dx
⇒∫d(y+2y)y+2y=∫dx⇒ ∫ \dfrac{d\left(y + {2}^{y}\right)}{y + {2}^{y}} = ∫ dx⇒∫y+2yd(y+2y)=∫dx
⇒ln∣y+2y∣=x+C[∵∫f′(x)f(x)dx=ln(f(x))+C]⇒ \ln \left|y + {2}^{y}\right| = x + C\left[∵ ∫ \dfrac{{f}^{'}\left(x\right)}{f\left(x\right)}dx = \ln \left(f\left(x\right)\right) + C\right]⇒ln∣y+2y∣=x+C[∵∫f(x)f′(x)dx=ln(f(x))+C]
Now ∵y(0)=0∵ y\left(0\right) = 0∵y(0)=0
⇒C=0⇒ C = 0⇒C=0
∴ln∣y+2y∣=x∴ \ln \left|y + {2}^{y}\right| = x∴ln∣y+2y∣=x
Now for y=1y = 1y=1 we have
x=ln(1+2)=ln3∈(1,2)x = \ln \left(1 + 2\right) = \ln 3 \in \left(1, 2\right)x=ln(1+2)=ln3∈(1,2)