Question

In an ap first term is 2 and common difference is 8 if sum of n terms is 90 then find value of n will be?

Answer 1

Answer:

n = 5

Step-by-step explanation:

a = 2, d = 8,. Sn = 90

Sn = n/2 (2a + (n-1)d)

$90 = \frac{n}{2} (2(2) + (n - 1)8) \\ 90 = \frac{n}{2} (4 + 8n - 8) \\ 90 \times 2 = n(8n - 4) \\ 180 = 8 {n}^{2} - 4n \\ 180 = 4( {2n}^{2} - n) \\ {2n}^{2} - n = \frac{180}{4} \\ 2 {n}^{2} - n = 45 \\ 2 {n}^{2} - n - 45 = 0 \\ 2 {n}^{2} - 10n + 9n - 45 = 0 \\ 2n(n - 5) + 9(n - 5) = 0 \\ (2n + 9)(n - 5) = 0 \\ 2n + 9 = 0 \: \: \: \: \: n - 5 = 0 \\ n = - \frac{9}{2} \: \: \: \: \: \: \: \: \: \: \: n = 5$

we take the positive value of n

therefore the sum of 5 terms is 90

2, 10, 18, 26, 34 .......

Answer 2

Answer:

answer of this question

Step-by-step explanation:

I hope it's correct