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gaobo9109
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Homework Statement
In an irregular quadrilateral ABCD, the length of all sides are AB=a BC=b CD=c DA=d and the length of the diagonal AC is x. Angle ABC + angle ADC = 180
prove that (ab+cd)x2=(ac+bd)(ad+bc)
Homework Equations
Cosine formula c2 = a2 + b2 - 2abcosθ
The Attempt at a Solution
I really have no idea how to start