Mathematics
The sum of n natural numbers is 5n2 + 4n. Find its 8th term.
AP
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Answer
Here T stands for term.
Sn = 5n2 + 4n
S1 = Sum of first natural number = T1
= 5(1)2 + 4(1) = 5 + 4 = 9.
S2 = 5(2)2 + 4(2) = 20 + 8 = 28.
T2 = S2 - S1 = 28 - 9 = 19.
We know that,
⇒ T2 = a + d
⇒ 19 = 9 + d
⇒ d = 10.
T8 = a + (8 - 1)d = 9 + 7(10) = 79.
Hence, 8th term = 79.
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