如下,郑旦点小拆带图,旅丛芦看大图
cos(A-C)+cosB=3/2cos(A-C)+cos(180-A-C)=3/2cos(A-C)—cos(A+C)=3/仿瞎猜2sinA * sinC = 3/4又神氏正弦定理:sinA=sinB/b*asinC=sinB/b*c上备型两式相乘后面你自己算吧