设an是一个公差为d（d不等与0）的等差数列,它的前n项和Sn=110且a1,a2,a4成等比数列,证明：a1=d

a2^2=a1a4(a1+d)^2=a1(a1+3d)a1^2+2a1d+d^2=a1^2+3a1da1d=d^2a1=da1=da2=d+d=2da3=d+2d=3d.an=a1+(n-1)d=d+nd-d=ndSn=d+2d+3d+.+nd=d(1+2+3.+n)=d(n+1)*n/2=110=nd(n+1)/2=110可能条件不全,公差d的值求不出来...