Question

15
How many terms of the
arithmetic progression
9, 17, 25, 33, ...
must be taken to get a
sum of 636?
​

Answer 1

Respuesta:

Approximately 12 - 13 term.

Explicación paso a paso:

1st term: 9     +8

2nd term: 9, 17        +8

3rd term: 9, 17, 25          +8

4th term: 9, 17, 25, 33         +8

r = 8

Formula:

An = A1 + (n-1).r                            

An = 9 + (n-1).8

An = 9 + 8n - 8

An = 8n + 1

Sn = (A1 + An) . n/2

Sn = (9 + (8n + 1) . n/2

$636 = \frac{(9 + 8n^{2} + n) }{2}\\\\ 1272 = 9 + 8n^{2} + n\\\\1263 = 8n^{2} + n$