In an arithmetic progression with first term a = 4 and common difference d = 3, the sum of the first 20 terms is:

解說

Use S_n = n/2 [2a + (n - 1)d] = 20/2 [8 + 19 x 3] = 10 x 65 = 650. The value 620 comes from using (n - 2) instead of (n - 1), and 1240 is the double.