Draw n straight lines in the circle. How many parts of the circle can you divide at most?

Draw n straight lines in the circle. How many parts of the circle can you divide at most?

A straight line is divided into two straight lines, four parts, three straight lines, seven parts, four straight lines, and eleven parts. The rule is to increase 2.3.4 each time. Because every time you have to intersect all the existing straight lines, the number of each increase is the n of the nth. In this way, you can use the sum of arithmetic sequence to calculate, the first time is regarded as 1 + 1, the second time as 1 + (1 + 2) the third time