(a十b十c)(a十b—c)等于ab ,求角C

题目

(a十b十c)(a十b—c)等于ab ,求角C

答案

(a+b)2-c2=ab
即a2+b2+2ab-c2=ab
a2+ b2+ab= c2
而c2= a2+ b-2absina
则ab=-2abcosa
cosa=-1/2
a=120