an/a(n- 1)=(n+ 1)/(n- 1)

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

...

a3/a2 = 4/2

a2/a 1 = 3/ 1

All multiplications a (n+1)/a1= (n+2) (n+1)/2 *1= (n+2) (n+1)/2.

So a (n+1) = (n+2) (n+1)/2.

an = n(n+ 1)/2