Question

Question 6
The partial sum 1+3+5+...+39 equals =

Answer 1

400

Explanation:

The 20 terms of the sum can be formed into 10 pairs, each with a sum of 40:

= (1 +39) +(3 +37) +(5 +35) + ... +(19 +21)

= 40 × 10 = 400

The indicated sum is 400.