The sum of the first 'm' terms of an AP is 4msquare - m.
if it's nth term is 107. Find the value of n and 21st term.

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Solution

S_m=4m²-m

a=s₁=4-1=3 (m=1)

s₂=14 (m=2)

s₂=a+a+d=14

d=8

n=107

a+(n-1)d=107

3+(n-1)8=107 (putting values of a & d)

n=14

a₂₁=a+20d

=3+20(8)

=163

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The sum of first m terms of an AP is $(4 m^{2} - m)$ . If its nth term is 107, find the value of n. Also, find the 21st term of this AP.

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The sum of first m terms of an A.P. is $4 m^{2}$ -m. If its nth terms is 107, find the value of n. Also, find the 21st term of this A.P.

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