Find sequence 1, 2, 4, 7, 11, 16 The general term formula of

Find sequence 1, 2, 4, 7, 11, 16 The general term formula of

a1=1
a(n)-a(n-1)=n-1
a(n-1)-a(n-2)=n-2


a2-a1=1
a(n)-a1=(1+n-1)*(n-1)/2
The rest is up to you