Question

The sum of n natural number is
5n^2 + 4n. Find the 8th term.
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Answer 1

Sn = 5n² + 4n

=> n/2 [ 2a + (n-1)d] = 5n²+ 4n

=> 2a +(n-1)d = 10n + 8

=> 2a +(n-1)d = 18 + 10n - 10

=> 2a +(n-1)d = 18 + 10(n-1)

Comparing both side, we get

a = 9

d = 10

T8 = 9 + ( 8-1) × 10

= 9 + 70

= 79

Answer 2

refer to the attached image :)