Question

Is part A correct? Im not entirely sure about it and also what is part B???

Answer 1

see explanation

Explanation:

A

Given

`S_(n)` =
`(1)/(2)`(n² + n) ← multiply each term in the parenthesis by
`(1)/(2)`

=
`(1)/(2)` n² +
`(1)/(2)` n

B

To find the sum of the first 12 whole numbers, substitute n = 12 into the formula, that is

`S_(12)` =
`(1)/(2)` (12² + 12) =
`(1)/(2)` (144 + 12) =
`(1)/(2)` × 156 = 78

Answer 2

A)
`(n^2)/(2) + (n)/(2)`

B) 78

Explanation:

B) To find the sum of first 'n' whole numbers , formula used is :-

`(1)/(2) ( n^2 + n)`

We have to find the sum of first 12 whole numbers . So here n = 12

Putting the value of 'n' in above formula ,

`(1)/(2) ( 12^2 + 12) \\\\=> (1)/(2) ( 144 + 12)\\\\=> (156)/(2) = 78`