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fie triunghiul ABC,AB=5cm,AC=6cm, BC=7cm cosA?

Răspuns :

7^2=5^2+6^2-2*5*6*cos(BAC)
49=25+36-60*cos(BAC)
49=61-60*cos(BAC)
60*cos(BAC)=61-49
60*cos(BAC)=12
cos(BAC)=12/60
cos(BAC)=1/5