Question

Find 1+2+3+4+ ... +40, the sum of the first 40 natural numbers.
The sum is

Answer 1

`\bold{S_(40)=820}`

Explanation:

`1+2+3+4+ ... +40=S_(40)\\\\a_1=1\\a_(40)=40\\n=40\\\\S_n=\frac{a_1+a_n}2\cdot n\\\\S_(40)=\frac{1+40}2\cdot40=41\cdot20=820`

Answer 2

Answer : 840

1 + 41 = 42

2 + 40 = 42

___________

20 pairs

20 × 42 = 840