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a²=b²+c²-2bc.cosα a=63N; b=45N; c=27N
(a²-b²-c²)/(-2bc)= cosα
(63²-45²-27²)/(-2(45)(27))=cosα
1215/(-2430)=cosα
-0.5=cosα
cos ^-1(-0.5)=α
120°=α