Question 1129844
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One way to do this is to split the quadrilateral into two pieces as shown below
<img src = "https://i.imgur.com/PhZMORb.png">
Recall that the sum of the three angles of any triangle is always 180 degrees. 
For each triangle, we can say
A+B+C = 180
D+E+F = 180
If we add up the two sides of the equations above, then we get
(A+B+C)+(D+E+F) = 180+180
(A+D)+B+(C+F)+E = 360
Which shows that the four angles add up to 360 regardless of what kind of quadrilateral we're working with
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