Question

How many
terms of Al :-2, 10, 18
must be taken to give the SIM of 90?
answer me step by step
I will brainlist there answer how will give me answer in step by step​

Answer 1

Step-by-step explanation:

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

Given :

Sn = 90

a = 2

d = 10-2 = 8

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

90 = n/2(2×2 + (n-1)8)

90 = n/2(4 + 8n - 8)

90 = n/2(8n - 4)

90×2 = n(8n - 4)

180 = n(8n - 4)

180 = 8n² - 4n

= 8n² - 4n - 180

= 2n² - n - 45. (dividing eqn. with 4)

= 2n² -10n + 9n - 45

= 2n(n - 5) + 9(n - 5)

= (2n + 9)(n - 5)

2n + 9 = 0

2n = -9

n = -9/2

n - 5 = 0

n = 5

So , The Answer is 5 (n can't be in negative value)

& The A.P. will be = 2, 10, 18, 26, 34

Answer 2

Answer:

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