2b = a+c.
c^2 = bd = (a+c)d/2,2/d = (a+c)/c^2
2/d = 1/c + 1/e = (a+c)/c^2 = a/c^2 + 1/c,
1/e = a/c^2,
c^2 = ae
a,c,e 成等比数列.