易知,2a,2b,2c成等差数列,===>2b=a+c.===>a²+2ac+c²=4b²=4(c²-a²).===>3c²-2ac-5a²=0.===>3e²-2e-5=0.===>e=5/3.（e=-1舍去).∴e=5/3.