a^2+c^2=b^2+aca^2+c^2-b^2=accosB=(a^2+c^2-b^2)/2ac=ac/2ac=1/2B=60°log4(sinA)+log4(sinC)=-1log4(sinAsinC)=-1sinAsinC=1/4-[cos(A+C)-cos(A-C)] /2=1/4cos(A+C)-cos(A-C)=-1/2cos(180-B)-cos(A-C)=-1/2-cosB-c...sinAsinC=1/4后面那步怎么来的？log4(sinAsinC)=-1积化和差 sinαsinβ=-[cos(α+β)-cos(α-β)] /2