Question

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

Answer 1

Answer : 840

1 + 41 = 42

2 + 40 = 42

___________

20 pairs

20 × 42 = 840

Answer 2

Answer:

              $\bold{S_{40}=820}$

Step-by-step explanation:

$1+2+3+4+ ... +40=S_{40}\\\\a_1=1\\a_{40}=40\\n=40\\\\S_n=\dfrac{a_1+a_n}2\cdot n\\\\S_{40}=\dfrac{1+40}2\cdot40=41\cdot20=820$