Explanation

Address the formula, input parameters & values.

Input parameters & values:

The number series 1, 3, 5, 7, 9, .  .  .  .  , 139.

The first term a = 1

The common difference d = 2

Total number of terms n = 70

step 2

apply the input parameter values in the AP formula

Sum = n/2 x (a + Tn)

= 70/2 x (1 + 139)

= (70 x 140)/ 2

= 9800/2

1 + 3 + 5 + 7 + 9 + .  .  .  .   + 139 = 4900

Therefore, 4900 is the sum of first 70 odd numbers.