If any quadrilateral ABCD, prove that `"sin"(A+B)+sin(C+D)=0` `"cos"(A+B)=cos(C+D)`

Explanation

To prove the given relations for any quadrilateral ABCD, we will follow these steps:

### Step 1: Understand the sum of angles in a quadrilateral
In any quadrilateral ABCD, the sum of the interior angles is given by:
\[ A + B + C + D = 2\pi \quad \text{(in radians)} \]
This means that if we know any three angles, we can find the fourth angle.
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