The first term of an arithmetic series is 4 and the common difference is 3. What is the sum of the first 20 terms?

解說

S_20 = 20/2 × [2(4) + 19(3)] = 10 × (8 + 57) = 650. The value 1240 drops the division by 2. The value 610 uses a in place of 2a inside the bracket.