(b-c)*cos²A=b*cos²B-c*cos²C,判断三角形ABC的形状.(b-c)cos^2_数学解答

(b-c)*cos²A=b*cos²B-c*cos²C,判断三角形ABC的形状.
(b-c)cos^2(π-(B+C))=bcos^2B-ccos^2C
(b-c)cos^2(B+C)=bcos^2B-ccos^2C
等式左边应该加上负号吧~

人气：216 ℃　时间：2020-05-03 10:28:30

解答

(b-c)cos^2A=bcos^2B-ccos^2C(b-c)cos^2(π-(B+C))=bcos^2B-ccos^2C(b-c)cos^2(B+C)=bcos^2B-ccos^2Cb(cos^2(B+C)-cos^2B)=c(cos^2(B+C)-cos^C)cos^2(B+C)-cos^2B=(cos(B+C)+cosB)(cos(B+C)-cosB)=2cos(2B+C)/2cosC/...