Are you the smartest #2

Algebra Level 4

The sum of the series

1 3 1 \dfrac{1^{3}}{1} + 1 3 + 2 3 1 + 3 \dfrac{1^{3}+2^{3}}{1+3} + 1 3 + 2 3 + 3 3 1 + 3 + 5 \dfrac{1^{3}+2^{3}+3^{3}} {1+3+5} ...............................

Upto 16 16 terms!!


The answer is 446.

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2 solutions

In this series, the sum of numerator of each term is ( n ( n + 1 ) / 2 ) 2 (n(n+1)/2) ^{2} and the sum of denominator of each term is n 2 . n^{2}.

So each term is ( n + 1 ) 2 / 4. (n+1)^{2}/4.

So the sum is

= ( 2 2 + 3 2 + . . . + 1 7 2 ) / 4 =(2^{2} +3^{2} + . . . + 17^{2})/4

= ( 1 2 + 2 2 + 3 2 + . . . + 1 7 2 1 ) / 4 =(1^{2} + 2^{2} +3^{2} + . . . + 17^{2} -1)/4

= [ ( 17 18 35 / 6 ) 1 ] / 4 =[(17*18*35/6)-1]/4

= 446 = \boxed{446}

Good job, sir. Well done.

Neeraj Snappy - 6 years, 6 months ago

Well done,sir.

Anik Mandal - 6 years, 5 months ago
Pranjal Jain
Dec 11, 2014

Let T r T_{r} denote the r t h r^{th} term of the series.

T r = 1 3 + 2 3 + 3 3 + . . . + r 3 1 + 3 + 5 + . . . + ( 2 r 1 ) T_{r}=\dfrac{1^{3}+2^{3}+3^{3}+...+r^{3}}{1+3+5+...+(2r-1)}

= m = 1 r m 3 m = 1 r ( 2 m 1 ) =\frac{\displaystyle\sum_{m=1}^{r} m^{3}}{\displaystyle\sum_{m=1}^{r} (2m-1)}

Denominator comes out to be r 2 r^{2} .

This can be proved by sum of an AP with initial term 1 1 , common difference 2 2 and number of terms r r .

Numerator comes out to be ( r ( r + 1 ) 2 ) 2 \bigg (\dfrac{r(r+1)}{2}\bigg )^{2} .

So T r = ( r ( r + 1 ) 2 ) 2 r 2 T_{r}=\dfrac{\Big (\frac{r(r+1)}{2}\Big )^{2}}{r^{2}}

= ( r + 1 ) 2 4 =\dfrac{(r+1)^{2}}{4}

S 16 = r = 1 16 ( r + 1 ) 2 4 S_{16}=\dfrac{\displaystyle\sum_{r=1}^{16} (r+1)^{2}}{4}

To ease up the question we can say that

S 16 = r = 2 17 r 2 4 S_{16}=\dfrac{\displaystyle\sum_{r=2}^{17} r^{2}}{4}

( r = 1 17 r 2 ) 1 2 4 \dfrac{\Big (\displaystyle\sum_{r=1}^{17} r^{2}\Big )-1^{2}}{4}

Now by using the formula that r = 1 n n 2 = n ( n + 1 ) ( 2 n + 1 ) 6 \displaystyle\sum_{r=1}^{n} n^{2}=\frac{n(n+1)(2n+1)}{6} ,

S S comes out to be 17 × 18 × 35 6 1 4 = 446 \dfrac{\frac{17×18×35}{6}-1}{4}=\boxed{446}

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