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Question
The sum of n natural numbers is 5n2 + 4n. Find its 8th term.
Solution
Sum of n natural number = 5n2 + 4n
Sn = 5n2 + 4n
S1 = (a) = 5 × (1)2 + 4 × 1
= 5 + 4
= 9
S2 = 5(2)2 + 4 × 2
= 20 + 8
= 28
S2 – S1 = T2
= 28 – 9
= 19
`=>` a + d = 19
`=>` 9 + d = 19
∴ d = 19 – 9 = 10
∴ a = 9, d = 10
∴ T8 = a + (n – 1)d
= 9 + (8 – 1) × 10
= 9 + 7 × 10
= 9 + 70
= 79
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