Differentiate w.r.t. x the function `cos (a cos x+ b sin x)`, for some constant a and b.

Explanation

To differentiate the function \( y = \cos(a \cos x + b \sin x) \) with respect to \( x \), we will use the chain rule. Here are the steps:

### Step 1: Identify the outer and inner functions
The outer function is \( \cos(u) \) where \( u = a \cos x + b \sin x \).

### Step 2: Differentiate the outer function
The derivative of \( \cos(u) \) with respect to \( u \) is:
\[
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