设公差为d则a3=a1+2d=1+2da9=a1+8d=1+8d因为a1,a3,a9成等比数列所以a3²=a1*a9=a9∴(1+2d)²=1+8d∴d=0或者d=1又∵d≠0,∴d=1所以an=a1+(n-1)d=n
