a4=2a2(a2+2d)=2a2a2=2da1=a2-d=d4=4a4=a2+2d=4d因为,a1 , 4 , a4成等比数列，所以，d(4d)=4^2d^2=4d=2a1=2a2=4d=a2-a1=2an=a1+(n-1)d=2+(n-1)2=2n
