a(n+1)=a(n)+2n = a(n) + n(n+1) - (n-1)n,
a(n+1) - n(n+1) = a(n) - (n-1)n = ...= a(1) - 0 = 2,
a(n) = 2 + (n-1)n,
a(100)=2+99*100=9902