Inspired by Rama Devi

Algebra Level 5

The sum of the series 60 + 660 + 6660 + . . . . . . . . . . . . . 60+660+6660+............. upto n n terms is given by e a ( b n + 1 c n d ) \large\frac{e}{a}(b^{n+1}-cn-d) .Find the value of a + b + c + d + e a+b+c+d+e .

a, b, c, d, e are positive integers. e and a are coprime.


The answer is 76.

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2 solutions

The r t h r^{th} term of the series is:

T r = 10 ( 6 ) ( 1 0 0 + 1 0 1 + . . . 1 0 r 1 ) T_r = 10(6)(10^0 + 10^1 + ... 10^{r-1})

T r = 60 i = 1 r 1 0 i 1 T_r = 60\displaystyle \sum_{i=1}^{r} 10^{i-1}

T r = 60 1 ( 1 0 r 1 ) 9 = 20 ( 1 0 r 1 ) 3 T_r = 60 \dfrac{1(10^r - 1)}{9} = \dfrac{20(10^r - 1)}{3}

The sum to n terms of the series is:

S = r = 1 n T r = r = 1 n 20 3 1 0 r r = 1 n 20 3 S = \displaystyle \sum_{r=1}^{n} T_r = \sum_{r=1}^{n} \dfrac{20}{3} 10^r - \sum_{r=1}^{n} \dfrac{20}{3}

S = 20 3 10 ( 1 0 n 1 ) 9 20 n 3 \Rightarrow S = \dfrac{20}{3} \dfrac{10(10^n - 1)}{9} - \dfrac{20n}{3}

S = 20 27 ( 1 0 n + 1 10 9 n ) \Rightarrow S = \dfrac{20}{27} (10^{n+1} - 10 - 9n)

e = 20 , a = 27 , b = 10 , c = 9 , d = 10 a + b + c + d + e = 76 e = 20 , a = 27 , b = 10 , c = 9 , d = 10 \Rightarrow a + b + c + d + e = \boxed{76}

Good solution

Sai Ram - 5 years, 10 months ago
Abhay Tiwari
Jun 10, 2016

60 ( 1 + 11 + 111 + ) 60(1+11+111+\dots)

60 9 ( 9 + 99 + 999 + ) \dfrac{60}{9}(9+99+999+\dots)

20 3 ( ( 10 1 ) + ( 100 1 ) + ( 1000 1 ) + ) \dfrac{20}{3}((10-1)+(100-1)+(1000-1)+\dots)

20 3 ( 10 + 100 + 1000 + + ( 10 ) n n \dfrac{20}{3}(10+100+1000+\dots+(10)^{n}-n

20 3 ( 10 ( ( 10 ) n 1 10 1 ) n \dfrac{20}{3}(10(\dfrac{(10)^{n}-1}{10-1})-n

20 27 ( ( 10 ) n + 1 10 9 n ) \dfrac{20}{27}((10)^{n+1}-10-9n)

comparing from the above mentioned form, we get a = 27 , b = 10 , c = 9 , d = 10 , e = 20 a=27,b=10, c=9, d=10, e=20

So, a + b + c + d + e = 76 a+b+c+d+e=\color{#D61F06}{\boxed{76}}

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