If the sum of the 33+73+113+153...................20 terms is S20. Find the value of S20100 .
Let S = 33+73+113+153+.........(1)
Above given series is sum of the cube of odd numbers starting from 3.
nth term of above given series
tn = [3+(n−1)×4]3 = [3+4n−4]3 = (4n−1)3
sum of the n terms
Sn=∑ni=1tn
= ∑ni=1 (4n−1)3
= ∑ni=1 [16n3−1−4n(4n−1)]
= ∑ni=1 [16n3−1−16n2+4n]
= 16 ∑ni=1n3 - ∑ni=11 - 16∑ni=1n2 + 4∑ni=1n
= 16[n(n+1)2]2 - n - 16[n(n+1)(2n+1)2] + 4 [n(n+1)2] ---------(2)
substitute n = 20 in equation (2)
=16[(20×21)2]2 - 20 - 16[(20×21×41)2] + 4 [(20×21)2]
=705600−2045920+840
S20=660500
S20100=6605