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a² = b²+c²-2bc cos A
a²-b² = c²-2bc cos A
di soal diketahui bahwa a²-b² = c²-bc, substitusikan:
c²-bc = c²-2bc cos A
-bc = -2bc cos A
cos A = -bc/-2bc
cos A = 1/2
A = arccos(1/2)
A = 60°