Differentiate the wave.

With x measured in radians, the derivative of $\sin x$ is $\cos x$ and that of $\cos x$ is $-\sin x$. A frequency multiplier inside the angle also multiplies the derivative. The minus sign for cosine matters.

Products such as $x\sin x$ need the product rule, and a quotient needs both numerator terms in the correct order. Simplifying a derivative can reveal stationary inputs or useful factors. Check a numerical gradient as a reasonableness test, not as a replacement for exact working.