Question

Find S5 for the sequence 3, 13, 23, 33, 43, 53, 63, 73.

A) 43
B) 115
C) 120
D) 225

Answer 1

The sequence is
3 , 13 , 23 , 33 , 43 , 53 , 63 , 73
S5 ⇒ The sum of the first 5 terms of the sequence
∴ S5 = 3 +13 + 23 + 33 + 43 = 115

So, the answer is option (B)
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Another solution:
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It is observed that the first number = 3
And each number increased by 10 from its previous number
So, the sequence is an arithmetic series.
let a ⇒ is the first term of an arithmetic sequence .
and d ⇒ is the difference between terms of the arithmetic sequence .
So Sn ⇒ is the sum of n terms in the arithmetic sequence .

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

for a = 3 , d = 10 , n = 5

Sn = (5/2)( 2*3 + 10 (5 -1) ) = 115 ⇒⇒⇒⇒⇒ option (B)