a(n) = a + (n-1)d,n = 1,2,...
[a(4)]^2 = [a(3) + d]^2 = (6+d)^2 = a(2)*a(8) = [a(3) - d][a(3) + 5d] = (6-d)(6+5d),
36 + 12d + d^2 = 36 + 24d - 5d^2,
0 = 6d^2 - 12d = 6d[d - 2],
d = 0 或 d = 2.
d=0时,6 = a(3)=a+2d=a.
a(n) = 6,
a(1)+a(2)+...+a(n)= 6n,n = 1,2,...
d=2时,6 = a(3)=a+2d=a+4,a = 2.
a(n)=2+2(n-1)=2n,n = 1,2,...
a(1)+a(2)+...+a(n)=n(n+1).n = 1,2,...