1 a1=2,a2= a1+c=2+c,a3=a2+2c=2+c+2c=2+3c.因a1,a2,a3成公比不为1的等比数列,所以a2^2=a1*a3,即(2+c)^2=2*(2+3c).整理得：c^2-2c=0,c（c-2）=0.所以c=0或c=2.因公比不为1,舍去c=0,于是有c=2.2 an+1=an+2n an+1-an=2...