s1=Sn-Sn-1=a(nT)^2/2-a{(n-1)T}^2/2=a(2n-1)T^2/2
s2=Sn+1 - Sn=a{(n+1)T}^2/2-a(nT)^2/2=a(2n+1)T^2/2
s3=Sn+2 - Sn+1=a{(n+2)T}^2/2-a{(n+1)T}^2/2=a(2n+3)T^2/2
s2-s1=aT^2
s3-s2=aT^2
所以 证得