Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
Let $$y = y(x)$$ be the solution of the differential equation $$dy = e^{\alpha x + y} dx$$; $$\alpha \in N$$. If $$y(\log_e 2) = \log_e 2$$ and $$y(0) = \log_e\left(\frac{1}{2}\right)$$, then the value of $$\alpha$$ is equal to _________.
Correct Answer: 2
We start with the differential equation
$$dy = e^{\alpha x + y}\,dx.$$
Dividing both sides by $$dx$$ gives
$$\frac{dy}{dx} = e^{\alpha x + y}.$$
Now separate the variables. We write the right‐hand side as a product:
$$\frac{dy}{dx} = e^{\alpha x}\,e^{y}.$$
Moving all terms in $$y$$ to the left and all terms in $$x$$ to the right gives
$$e^{-y}\,dy = e^{\alpha x}\,dx.$$
Next we integrate both sides. For the left side we recall the basic integral
$$\int e^{-y}\,dy = -e^{-y} + C_1,$$
and for the right side we recall
$$\int e^{\alpha x}\,dx = \frac{1}{\alpha}e^{\alpha x} + C_2,$$
where $$C_1$$ and $$C_2$$ are constants of integration. Absorbing the two constants into one overall constant $$C$$, we have
$$-e^{-y} = \frac{1}{\alpha}e^{\alpha x} + C.$$
Multiplying through by $$-1$$ simplifies the expression:
$$e^{-y} = -\frac{1}{\alpha}e^{\alpha x} + C'.$$
Renaming the constant $$C'$$ as $$C$$ for convenience, we obtain the general implicit solution
$$e^{-y} = C - \frac{1}{\alpha}e^{\alpha x}. \quad -(1)$$
We now use the two given initial conditions to determine the constant $$C$$ and then find $$\alpha$$.
First condition $$y(0)=\ln\!\left(\dfrac12\right).$$
At $$x=0$$ we have $$y = \ln\!\left(\dfrac12\right) = -\ln 2.$$ Thus
$$e^{-y} = e^{-\bigl(-\ln 2\bigr)} = e^{\ln 2} = 2,$$
and
$$e^{\alpha x} = e^{\alpha\cdot 0} = 1.$$
Substituting these values into (1) gives
$$2 = C - \frac{1}{\alpha}\cdot 1,$$
so that
$$C = 2 + \frac{1}{\alpha}. \quad -(2)$$
Second condition $$y(\ln 2)=\ln 2.$$
At $$x = \ln 2$$ we have $$y = \ln 2,$$ hence
$$e^{-y} = e^{-\ln 2} = \frac12,$$
and
$$e^{\alpha x} = e^{\alpha\ln 2} = 2^{\alpha}.$$
Substituting into (1) and using the value of $$C$$ from (2) we get
$$\frac12 = \left(2 + \frac{1}{\alpha}\right) - \frac{1}{\alpha}2^{\alpha}.$$
Simplifying step by step:
$$\frac12 = 2 + \frac{1}{\alpha} - \frac{2^{\alpha}}{\alpha},$$
$$\frac12 - 2 = \frac{1 - 2^{\alpha}}{\alpha},$$
$$-\frac32 = \frac{1 - 2^{\alpha}}{\alpha}.$$
Multiplying both sides by $$\alpha$$ yields
$$-\frac{3\alpha}{2} = 1 - 2^{\alpha},$$
which can be rearranged as
$$2^{\alpha} - 1 = \frac{3\alpha}{2}. \quad -(3)$$
Because $$\alpha$$ is a natural number, we test successive integers in equation (3).
For $$\alpha = 1$$: the left side is $$2^{1}-1 = 1,$$ while the right side is $$\frac{3\cdot1}{2} = 1.5,$$ which is not equal.
For $$\alpha = 2$$: the left side is $$2^{2}-1 = 3,$$ while the right side is $$\frac{3\cdot2}{2} = 3,$$ which matches exactly.
Hence the natural number $$\alpha$$ satisfying all the given conditions is $$\alpha = 2.$$
So, the answer is $$2$$.
Create a FREE account and get:
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.