sinC=sin2A=2sinAcosA=2sinA×(3/4)=(3/2)sinA
a/sinA=c/sinC=c/[(3/2)sinA]
∴c=3a/2
又∵a+c=10
∴a=4,c=6
cosA=(b²+c²-a²)/(2bc)=3/4
∴b=4或5