Given four points a (- 1,3), B (1,1), C (4,4) d (3,5), this paper proves that the quadrilateral ABCD is a trapezoid, using sine and cosine theorem~
Using vector to do the simplest! To do it with sine and cosine theorem, we need to take the midpoint e of AB, and use cosine theorem to prove that cos angle CDE is equal to Cos angle DEA, so that the angle CDE is equal to angle DEA, and there is ab parallel CD!
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