等差数列An中：A2-A1=d=8所以：公差d=8A4是A2和A3的等比中项：(A1+3d)^2=(A1+d)*(A1+2d)(A1+24)^2=(A1+8)(A1+16)A1^2+48A1+576=A1^2+24A1+12824A1=128-576=-448A1=-56/3Sn=nA1+n(n-1)d/2=-56n/3+4n(n-1)=4n^2-4n-56n/3=4n^2-68n/3
