A=45° 得 sinA=cosA=√2/2cosC=4/5>0 知 0°sinB=sin[180°-(A+C)]=sin(A+C)=sinAcosC+cosAsinC=√2/2×(4/5+3/5)=7√2/10正弦定理：a/sinA=b/sinB=c/sinC 即 a/(√2/2)=√2/(7√2/10)=c/(3/5)得 a=5√2/7，c=6/7
