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a²=b²+c²-2bc cosA, dimana adalah sudut diapit b dan c
a²-b²-c²-bc=0
subsitusi dr rumus umum ... b²+c²-2bc CosA -b²-c²-bc=0
menjadi -2bc Cos A - bc=0 .....menjadi CosA= - 1/2
maka A = 60 derajad