The first graph has three triangles, the second graph has six triangles, the third graph has ten triangles, the fourth graph has fifteen triangles, and the nth graph has several triangles? This topic is adapted... Can't do it

The first graph has three triangles, the second graph has six triangles, the third graph has ten triangles, the fourth graph has fifteen triangles, and the nth graph has several triangles? This topic is adapted... Can't do it

Hello, landlord
The first graph has 3 = 1 + 2 triangles
The second graph has 6 = 1 + 2 + 3 triangles
The third graph has 10 = 1 + 2 + 3 + 4 triangles
The fourth graph has 15 = 1 + 2 + 3 + 4 + 5 triangles
So the nth graph has 1 + 2 + 3 + +(n + 1) = (1 + N + 1) (n + 1) / 2 = [(n + 2) (n + 1) / 2] triangles
I hope you are satisfied