Question

In an AP given a=2,d=8,sn=90 find n and An

Answer 1

Hi Friend !!

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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 = 8n²- 4n

8n²- 4n-180 = 0

4(2n²- n - 45) = 0

2n²- n-45 = 0

2n²-7n+5n-45 = 0

n(2n-7)+5(2n-7) = 0

(n+5)(2n-7) =0

n = -5 or 7/2

Hope it helps u

Answer 2

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

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

180=n(4+8n-8)

180=n(8n-4)

180=8n square - 4n

8n square - 4n-180=0

After Solving this equation....

N=-5
N=7/2