The sum of the series
1 1 3 + 1 + 3 1 3 + 2 3 + 1 + 3 + 5 1 3 + 2 3 + 3 3 ...............................
Upto 1 6 terms!!
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Good job, sir. Well done.
Well done,sir.
Let T r denote the r t h term of the series.
T r = 1 + 3 + 5 + . . . + ( 2 r − 1 ) 1 3 + 2 3 + 3 3 + . . . + r 3
= m = 1 ∑ r ( 2 m − 1 ) m = 1 ∑ r m 3
Denominator comes out to be r 2 .
This can be proved by sum of an AP with initial term 1 , common difference 2 and number of terms r .
Numerator comes out to be ( 2 r ( r + 1 ) ) 2 .
So T r = r 2 ( 2 r ( r + 1 ) ) 2
= 4 ( r + 1 ) 2
S 1 6 = 4 r = 1 ∑ 1 6 ( r + 1 ) 2
To ease up the question we can say that
S 1 6 = 4 r = 2 ∑ 1 7 r 2
4 ( r = 1 ∑ 1 7 r 2 ) − 1 2
Now by using the formula that r = 1 ∑ n n 2 = 6 n ( n + 1 ) ( 2 n + 1 ) ,
S comes out to be 4 6 1 7 × 1 8 × 3 5 − 1 = 4 4 6
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In this series, the sum of numerator of each term is ( n ( n + 1 ) / 2 ) 2 and the sum of denominator of each term is n 2 .
So each term is ( n + 1 ) 2 / 4 .
So the sum is
= ( 2 2 + 3 2 + . . . + 1 7 2 ) / 4
= ( 1 2 + 2 2 + 3 2 + . . . + 1 7 2 − 1 ) / 4
= [ ( 1 7 ∗ 1 8 ∗ 3 5 / 6 ) − 1 ] / 4
= 4 4 6