Nova (series and sequences)

[ma200]

Let U represent the sum of integers from 1 to 20 and let V represent the sum of integers from 21 to 40. What is the value of V-U?

OA: 400

[Md. Minuddin]

Difference between the corresponding term of the two series is 20

So difference between the sum of two series is 20*20=400

Or
We know sum of first n natural number = n*(n+1)/2
So,V=40*41/2-20*21/2=820-210=610
Hence, V-U=610-20*21/2=610-210=400