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Let a1, a2, a3, …… , a100 be an arithmetic progression with a1 = 3 and SP = Σai, where the summation runs over p and 1 ≤ p ≤ 100. For any integer n with 1 ≤ n ≤ 20, let m = 5n. What is the value of a2 if Sm/ Sndoes not depend on n.
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Answer:
a₂ = 3 or 9
Step-by-step explanation:
Sm = (m/2)(2a + (m - 1)d)
Sm = (5n/2)(2*3 + (5n-1)d)
Sm = (5n/2) ( 6 - d + 5nd)
Sn = (n/2)(2a + (n - 1)d)
Sn= (n/2)(6 -d + nd )
Sm /Sn = (5n/2) ( 6 - d + 5nd) / ((n/2)(6 -d + nd )
= 5 (6 - d + 5nd) /(6 - d + nd)
Let say 5 (6 - d + 5nd) /(6 - d + nd) = k
=> 30 - 5d + 25nd = 6k - kd + knd
=> nd(25 - k) = d(5- k) - 6(5 - k)
=> nd(25 - k) = (d - 6)(5 - k)
as these are independnt of n , hence
d = 0 , k = 25 , d = 6 , k = 5 are the solutions
d = 0 , d = 6
a₂ = a₁ + d
a₁ = 3
a₂ = 3 + 0 or 3 + 6
a₂ = 3 or 9
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