设三边abc,对应角ABC,则面积为S=1/2bcsinA；（已知a,A）由正玄定理得到a/sinA=b/sinB=c/sinC,则：S=1/2bcsinA=a^2sinBsinC/2sinA=(a^2/2sinA){(cos(B-C)-cos(B+C)}/2=(a^2/4sinA){(cos(B-C)+cos(A)}《(a^2/4sinA){(1...