翔宇2006
幼苗
共回答了11个问题采纳率:90.9% 举报
-b/(2a+c)=cosB/cosC=[(c?+a?-b?)/(2ca)]/[(a?+b?-c?)/(2ab)]=[b(c?+a?-b?)]/[c(a?+b?-c?)]c?-a?c-b?c=2ac?+2a?-2ab?+c?+a?c-b?c,0=a?+a?c-ab?+ac?=a[(c?+a?-b?)+ac]-ac=c?+a?-b?=2accosB,cosB=-1/2=cos120°,B=120°13=b?=a?+ac+c?=(a+c)?-ac=4?-ac,ac=3,S⊿=acsinB/2=3sin120°/2=3√3/4
1年前
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