Question

.If a1, a2, . . ., an, . . . form an AP where an is defined as below an = 3 + 4n.Find the sum of the first 25 terms .
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Answer 1

a= 4n +3

substitute n=1

a = 4+3

= 7

a means first term

second term is

a2 = 4(2)+3

= 11

we know that common difference is a2- a1 11-7

= 4

sn = n/2 [ 2a + (n-1) d ]

here , a = 7 , d= 4 , n= 25

sn = 25/2 [ 2(7) + 24(4) ]

= 25/2 × 110

= 1,375

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