Question

The sum of first m terms of an AP is 4m 2 -m. if its n th term is 107, find the value of n. also find the 21 st term of this AP.

Answer 1

Given Sm=4m^2-m an=107---> a+(n-1)d=107 S1=4*1-1=3 S2=4*4-2=14 S2-S1=a2=11 a+d=11. [[[ d=8]]] a=11-d =11-8=3 a+8n=115. 3+8n=115. n=112/8=14 a21=a+20d = 3+20*8 =163

Answer 2

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