瞬閒美麗
幼苗
共回答了19个问题采纳率:84.2% 举报
4sinC+4sinA=5sinB+5sinC ==>sinC=4sinA-5sinB (1)
6sinC+6sinA=5sinA+5sinB (2)
(1)+(2) ==>7sinC+6sinA=9sinA ==>sinA=7/3sinC
再带入(1)求出sinA=7/5sinB ==>sinA:sinB:sinC=7:5:3 所以A最大,再根据正弦定理sinA=a/(2r),sinB=b/(2r),sinC=c/(2r) ==>a:b:c=7:5:3 ,根据余弦定理cosA=(b^2+c^2-a^2)/(2bc) =(25+9-49)/(2*5*3) =-15/30 =-1/2 ==>A=120.
1年前
2