Not so standard binomial

Algebra Level 4

7 2 × 3 ( 1 3 ) + 9 3 × 4 ( 1 3 ) 2 + 11 4 × 5 ( 1 3 ) 3 + \frac { 7 }{ 2\times 3 } \left( \frac { 1 }{ 3 } \right) +\frac { 9 }{ 3\times 4 } { \left( \frac { 1 }{ 3 } \right) }^{ 2 }+\frac { 11 }{ 4\times 5 } { \left( \frac { 1 }{ 3 } \right) }^{ 3 }+\ldots

The sum of the expression above up to 10 terms equal to?

1 3 1 12 × 3 10 \frac { 1 }{ 3 } -\frac { 1 }{ 12\times { 3 }^{ 10 } } 1 2 1 12 × 3 10 \frac { 1 }{ 2 } -\frac { 1 }{ 12\times { 3 }^{ 10 } } 1 2 1 10 × 3 10 \frac { 1 }{ 2 } -\frac { 1 }{ 10\times { 3 }^{ 10 } } 1 2 + 1 12 × 3 10 \frac { 1 }{ 2 } +\frac { 1 }{ 12\times { 3 }^{ 10 } }

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1 solution

Vighnesh Raut
May 11, 2015

The general term of the series can be written as ( 5 + 2 i ) ( i + 1 ) ( i + 2 ) ( 1 3 ) i \frac { \left( 5+2i \right) }{ (i+1)(i+2) } { \left( \frac { 1 }{ 3 } \right) }^{ i } Let us name the summation as S S . So, S = i = 1 10 ( 5 + 2 i ) ( i + 1 ) ( i + 2 ) ( 1 3 ) i S=\sum _{ i=1 }^{ 10 }{ \frac { \left( 5+2i \right) }{ (i+1)(i+2) } { \left( \frac { 1 }{ 3 } \right) }^{ i } } S = i = 1 10 ( 3 ( i + 1 ) 1 ( i + 2 ) ) ( 1 3 ) i S=\sum _{ i=1 }^{ 10 }{ \left( \frac { 3 }{ (i+1) } -\frac { 1 }{ (i+2) } \right) { \left( \frac { 1 }{ 3 } \right) }^{ i } } = 1 2 + ( 1 3 ( 3 ) + 1 3 ( 3 ) ) + ( 1 4 ( 3 2 ) + 1 4 ( 3 2 ) ) . . . . . . . . . . . + ( 1 11 ( 3 9 ) + 1 11 ( 3 9 ) ) 1 12 ( 3 10 ) S = 1 2 1 12 × 3 10 =\frac { 1 }{ 2 } +\left( -\frac { 1 }{ 3(3) } +\frac { 1 }{ 3(3) } \right) +\left( -\frac { 1 }{ 4({ 3 }^{ 2 }) } +\frac { 1 }{ 4({ 3 }^{ 2 }) } \right) ...........+\left( -\frac { 1 }{ 11({ 3 }^{ 9 }) } +\frac { 1 }{ 11({ 3 }^{ 9 }) } \right) -\frac { 1 }{ 12({ 3 }^{ 10 }) } \\ S=\frac { 1 }{ 2 } -\frac { 1 }{ 12{ \times 3 }^{ 10 } }

Moderator note:

Nicely done. Although it would be clearer to input parenthesis of pairs of terms that would be canceled out pairwise.

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