设首项为a1a3=a1+2d a9=a1+8da1，a3，a9成等比数列所以(a3)²=a1a9即(a1+2d)²=a1(a1+8d)整理的：d²=a1d因为d不等于0 所以a1=d则 a3=3d a9=9d所以公比是3
