b²+c²=a²+bc,bc=b²+c²-a²cosA=（b²+c²-a²)/2bc=bc/2bc=1/2A=60度sin²B+sin²C=1,1/2(1-cos2B)+1/2(1-cos2C)=1cos2B+cos2C=0cos(B+C)cos(B-C)=0cosAcos(B-C)=...搞错了，sin²B/+sin²C/=1应是sin²B/2+sin²C/2=1in²B/2+sin²C/2=11/2(1-cosB)+1/2(1-cosC)=1cosB+cosC=0cos(B+C)/2cos(B-C)/2=0sinA/2cos(B-C)/2=0sin30cos(B-C)/2=0cos(B-C)/2=0B=C,A=60度三角形ABC是等边三角形