a2-a1=2a3-a2=3a4-a3=4a5-a4=5a6-a5=6..an-a(n-1)=n以上等式相加得a2-a1+(a3-a2)+(a4-a3)+(a5-a4)+(a6-a5)+..+(an-a(n-1))=2+3+4+..+nan-a1=(n+2)(n-1)/2an=(n^2+n)/2 ∴sn=[(1^2+2^2+……n^2)+(1+2+……+n)]/2=[n*(...