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3.3k views
in Arithmetic Progression by (250 points)
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The sum of n natural numbers is 5n2 + 4n. Find it's 8th term.

2 Answers

+1 vote
by (45 points)
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Best answer
Sn=5n2+4n

S1=5(1)^2+4(1)

=5+4

=9

Therefore a1=s1=9

S2=5(2)^2+4(2)

=5(4)+8

=28

a2=S2-S1

a2=28-9=19

D=10

A8=a+7d

=9+7(10)

=9+70

=79
0 votes
by (29.3k points)

Sum of n natural numbers S= 5n2 + 4n

Sum of (n - 1) natural numbers

8th term = t8 = 10 x 8 - 1 = 80 - 1 = 79

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