Question

(vi) given a = 2, d=8, Sn= 90, find n and an .
Please answer the question quickly all my dear friends ​

Answer 1

a = 2

d = 8

Sn = 90

Sn = n/2 [ 2a + ( n – 1 )d ]

90 = n/2 [ 2×2 + ( n – 1 ) 8 ]

90 = n/2 × 2 [ 2 + 4n – 4 ]

90 = n [ – 2 + 4n ]

45 = n [ – 1 + 2n ]

45 = – n + 2n²

2n² – n – 45 = 0

2n² –10n + 9n – 45 = 0

2n ( n – 5 ) + 9 ( n – 5 ) = 0

( n – 5 ) ( 2n + 9 ) = 0

n = 5 [ n ≠ –9/2 since no of terms cannot be negative ]

an = a + ( n – 1 ) d

an = 2 + 4d

an = 2 + 4 × 8

an = 2 + 32

an = 34

Answer 2

Step-by-step explanation:

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