Practicing Success
If $y=e^{-x}(A \cos x+B \sin x)$, then y satisfies |
$\frac{d^2 y}{d x^2}+2 \frac{d y}{d x}=0$ $\frac{d^2 y}{d x^2}-2 \frac{d y}{d x}+2 y=0$ $\frac{d^2 y}{d x^2}+2 \frac{d y}{d x}+2 y=0$ $\frac{d^2 y}{d x^2}+2 y=0$ |
$\frac{d^2 y}{d x^2}+2 \frac{d y}{d x}+2 y=0$ |
$y=e^{-x}(A \cos x+B \sin x)$ .......(1) $\Rightarrow \frac{d y}{d x}=e^{-x}(-A \sin x+B \cos x)-e^{-x}(A \cos x+B \sin x)$ $\Rightarrow \frac{d y}{d x}=e^{-x}(-A \sin x+B \cos x)-y$ .......(2) $\Rightarrow \frac{d^2 y}{d x^2}=e^{-x}(-A \cos x-B \sin x)-e^{-x}(-A \sin x+B \cos x)\left(-\frac{d y}{d x}\right)$ Using (1) and (2), we get $\frac{d^2 y}{d x^2}+\frac{d y}{d x}=-y-y-\frac{d y}{d x}$ Hence (3) is the correct answer. |