解ï¼
1ãAãBãCæçå·®æ°åï¼æ以2B=A+C
åA+B+C=180ï¼æ以B=60 A+C=120
å 为b^2=a^2+c^2-2accosB
æ以13=9+c^2-2*3*ccos60
å³c^2-3c-4=0
å³c=4(c=-1èå»ï¼
2ãt=sinAsinC=[cos(A-C)-cos(A+C)]/2
=[cos(A-C)-cos120]/2
=[cos(A-C)+1/2]/2
=cos(A-C)/2+1/4
å½cos(A-C)=1æ¶å³A=B=60æ¶cos(A-C)/2+1/4æ大å¼=3/4
å³t=sinAsinCæ大å¼=3/4
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