1 right11is actually equal to1+11. So if you take each item apart, it becomes (1+3 ...+19)+(111+2/1+ ...

Arithmetic progression's method is used in brackets. Simply put, the beginning and the end are all equal to the same value, such as 1+ 19 = 20, 3+ 17 = 20, 5+ 15 = 20 ... The sum of * * is five equal values.