1+1=2,2+2=4,4+3=7.
a1=1,a2=a1+1,a3=a2+2.
an=a(n-1)+(n-1)
=a(n-2)+(n-2)+(n-1)
=a(n-3)+(n-3)+(n-2)+(n-1)
-----------------------
=a1+1+2+3+...+(n-1)
=1+n(n-1)/2