Step 1: This is a arithmetic progression. You can add a graph (a point) in front of it, so the law is obvious: +2, +4, +6, +8. ...

Step 2: Divide them all by two to become+1, +2, +3, +4 ... until they are added to the nth one.

Step 3: This part can be written as (n+ 1)*n/2.

Step 4: Because the previous division was divided by two, now multiply by two, (n+ 1)*n/2*2, and throw away the following /2*2. The formula is (n+1) * n.

Step 5: Add a center point to make it (n+ 1)*n+ 1.

Attachment: When the number is 3,7,13,21... this is (n+ 1)*n+ 1, (it can also be written as the square of n +n+ 1), but if it is/,

I hope I can help you.