Prove that `f(x)=a x+b` , where `a ,\ b` are constants and `a<0` is a decreasing function on `R` .

Explanation

To prove that the function \( f(x) = ax + b \) is a decreasing function on \( \mathbb{R} \) when \( a < 0 \), we will follow these steps:

### Step 1: Differentiate the function
We start by finding the derivative of the function \( f(x) \).

\[ f'(x) = \frac{d}{dx}(ax + b) \]
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