首先,在△ABC中,A+B+C=180度
那么cosB=-cos(A+C),即原式为
cos2B-cos(A+C)+cos(A-C)=1;
由二倍角公式及和差化积公式,得
cos2B=1-2(sinB)^2;
cos(A-C)-cos(A+C)=2sinAsinC;
即原式变为
1-2(sinB)^2+2sinAsinC=1;
即sinAsinC=(sinB)^2;
再由正弦定理sinA/a=sinB/b=sinC/c得
ac=b^2;
因此a.b.c成等比数列
Do you understand?
看来我得给你说明下:
cosa-cosb=-2sin1/2(a+b)*sin1/2(a-b)
cos(A-C)-cos(A+C)=-2sin(2A/2)sin(-2C/2)=2sinAsinC