In fact, when we cut, we will find that after the nth straight line is cut, there will be n more copies.

Namely:

F(n)-F(n- 1)=n

F(n- 1)-F(n-2)=n- 1

...

F(2)-F( 1)=2

Cumulative:

F(n)-F( 1)=2+3+...+n=(n+2)(n- 1)/2=(n？ +n-2)/2=n(n+ 1)/2- 1

F( 1)=2。

F(n)=n(n+ 1)/2+ 1