If $\frac{dv}{dx} = y + 3 > 0$ and $y(0) = 2$, then $y(\ln 2)$ is equal to
A-2
B7
C5
D13
Answer & Solution
Correct answer: B. 7
There is likely a typo in the differential equation, and the intended equation is $\frac{dy}{dx}=y+3$, since the condition uses $y(0)$ and asks for $y(\ln 2)$. Then solve it as a first-order linear equation.
$$\frac{dy}{dx}-y=3$$
A constant particular solution gives $y=-3$, and the complementary solution is $y=Ce^x$. So
$$y=Ce^x-3$$
Using $y(0)=2$,
$$2=C-3$$
$$C=5$$
Hence
$$y=5e^x-3$$
Now evaluate at $x=\ln 2$:
$$y(\ln 2)=5e^{\ln 2}-3$$
$$y(\ln 2)=5\cdot 2-3$$
$$y(\ln 2)=7$$
On checking the options, this matches option $\text{B}$.
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