Question

- The sum of first m terms of an AP is (4m2 - m). If its nth term is 107, find
the value of n. Also, find the 21st term of this AP.
[CBSE 2013]
ao find​

Answer 1

Answer:

Explanation:

Sn = 4m²-m

a1 = S1 = 4(1)²-(1) = 3

a2 = S2-S1 = 4(2)²-2-3 = 16-5 = 11

d = a2-a1 = 11-3 = 8

nth term is 107 (given)

Tn = 107

a1+(n-1)d = 107

3+(n-1)8 = 107

n-1 = 104/8 = 13

=> n = 14

Now,

21st term

T20 = a1+20d

= 3+20×8 = 3 + 160 = 163

Hope it helps!

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