Let $y=m \sin r x+n \cos r x$. What is the value of $\frac{d^2 y}{d x^2}$ ?

$r y$ $-r y$ $r^2 y$ $-r^2 y$

$-r^2 y$

$y=m \sin r x+n \cos r x$ $\frac{dy}{dx} = mr \cos rx - nr \sin rx$ $\frac{d^2 y}{d x^2} = -mr^2 \sin rx - nr^2 \cos rx = -r^2y$ Option: 4