The sum of the series 6 0 + 6 6 0 + 6 6 6 0 + . . . . . . . . . . . . . upto n terms is given by a e ( b n + 1 − c n − d ) .Find the value of a + b + c + d + e .
a, b, c, d, e are positive integers. e and a are coprime.
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Good solution
6 0 ( 1 + 1 1 + 1 1 1 + … )
9 6 0 ( 9 + 9 9 + 9 9 9 + … )
3 2 0 ( ( 1 0 − 1 ) + ( 1 0 0 − 1 ) + ( 1 0 0 0 − 1 ) + … )
3 2 0 ( 1 0 + 1 0 0 + 1 0 0 0 + ⋯ + ( 1 0 ) n − n
3 2 0 ( 1 0 ( 1 0 − 1 ( 1 0 ) n − 1 ) − n
2 7 2 0 ( ( 1 0 ) n + 1 − 1 0 − 9 n )
comparing from the above mentioned form, we get a = 2 7 , b = 1 0 , c = 9 , d = 1 0 , e = 2 0
So, a + b + c + d + e = 7 6
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The r t h term of the series is:
T r = 1 0 ( 6 ) ( 1 0 0 + 1 0 1 + . . . 1 0 r − 1 )
T r = 6 0 i = 1 ∑ r 1 0 i − 1
T r = 6 0 9 1 ( 1 0 r − 1 ) = 3 2 0 ( 1 0 r − 1 )
The sum to n terms of the series is:
S = r = 1 ∑ n T r = r = 1 ∑ n 3 2 0 1 0 r − r = 1 ∑ n 3 2 0
⇒ S = 3 2 0 9 1 0 ( 1 0 n − 1 ) − 3 2 0 n
⇒ S = 2 7 2 0 ( 1 0 n + 1 − 1 0 − 9 n )
e = 2 0 , a = 2 7 , b = 1 0 , c = 9 , d = 1 0 ⇒ a + b + c + d + e = 7 6