2 × 3 7 ( 3 1 ) + 3 × 4 9 ( 3 1 ) 2 + 4 × 5 1 1 ( 3 1 ) 3 + …
The sum of the expression above up to 10 terms equal to?
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Nicely done. Although it would be clearer to input parenthesis of pairs of terms that would be canceled out pairwise.
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The general term of the series can be written as ( i + 1 ) ( i + 2 ) ( 5 + 2 i ) ( 3 1 ) i Let us name the summation as S . So, S = i = 1 ∑ 1 0 ( i + 1 ) ( i + 2 ) ( 5 + 2 i ) ( 3 1 ) i S = i = 1 ∑ 1 0 ( ( i + 1 ) 3 − ( i + 2 ) 1 ) ( 3 1 ) i = 2 1 + ( − 3 ( 3 ) 1 + 3 ( 3 ) 1 ) + ( − 4 ( 3 2 ) 1 + 4 ( 3 2 ) 1 ) . . . . . . . . . . . + ( − 1 1 ( 3 9 ) 1 + 1 1 ( 3 9 ) 1 ) − 1 2 ( 3 1 0 ) 1 S = 2 1 − 1 2 × 3 1 0 1