Finding the open intervals seems easy to me except when I stumble upon trigonometry. 
**Problem:**

> 
> identify intervals in which the function is increasing or decreasing 
>  **f(x)=sin(x)+cos(x) $[0,2 \pi]$**
> **My attempt**
>  I started with the derivative which is f'(x)=cos(x)-sin(x) then equaled it to zero. I'm confused as to what to do with $[0,2\pi]$
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