Healthy Algorithm

Algebra Level 5

1 3 2 3 3 3 4 3 5 3 6 3 7 3 8 3 9 3 1 0 3 1 1 3 1 2 3 1 3 3 1 4 3 1 5 3 {1^3 \\ 2^3 \quad 3^3 \\ 4^3 \quad 5^3 \quad 6^3 \\ 7^3 \quad 8^3 \quad 9^3 \quad 10^3 \\ 11^3 \quad 12^3 \quad 13^3 \quad 14^3 \quad 15^3 \\ \vdots }

Find the sum of numbers in the 4 5 th 45^\text{th} row.


The answer is 46800979875.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Rohit Ner
Jun 5, 2015

First of all we need to determine the first number of the 45 t h {45}^{th} row. The sequence of first numbers of each row is: 1 , 2 , 4 , 7 , 11.. 1,2,4,7,11 . . We can observe that the second order difference i.e. difference of differences is a constant value.
So lets consider a quadratic expression P ( n ) : a n 2 + b n + c P(n):a{ n }^{ 2 }+bn+c
P ( 1 ) : a + b + c = 1 P ( 2 ) : 4 a + 2 b + c = 2 P ( 3 ) : 9 a + 3 b + c = 4 P(1):a+b+c=1\\P(2):4a+2b+c=2\\P(3):9a+3b+c=4
Solving the system of equations, we obtain a = 1 2 , b = 1 2 , c = 1 a=\frac{1}{2},b=\frac{-1}{2},c=1 P ( n ) : 1 2 n 2 1 2 n + 1 P ( 45 ) = 991 \Rightarrow P(n):\frac{1}{2}{ n }^{ 2 }-\frac{1}{2}n+1\\\Rightarrow P(45)=991 .
The last number of the 45 t h {45}^{th} row will be 991 + 44 = 1035 991+44=1035 .
Hence the 45 t h {45}^{th} row is: 991 3 , 992 3 , 993 3 . . . 1035 3 = r = 1 1035 r 3 r = 1 990 r 3 = ( 1035 × 1036 2 ) 2 ( 991 × 992 2 ) 2 = 287435376900 240634397025 = 46800979875 {991}^{3},{992}^{3},{993}^{3} . . . {1035}^{3}\\=\displaystyle\sum _{ r=1 }^{ 1035 }{ { r }^{ 3 } }-\displaystyle\sum _{ r=1 }^{ 990 }{ { r }^{ 3 } }\\={ \left( \frac { 1035\times 1036 }{ 2 } \right) }^{ 2 }-{ \left( \frac { 991\times 992 }{ 2 } \right) }^{ 2 }\\=287435376900-240634397025\\\Huge\color{#3D99F6}{=\boxed{46800979875}}


Nice Solution.

Sandeep Bhardwaj - 6 years ago

Log in to reply

Thank you sir. :)

Rohit Ner - 6 years ago

It would be better if you mention why you have chosen general term of the sequence ( 1 , 2 , 4 , 7 , 11 , ) (1,2,4,7,11, \cdots ) to be a quadratic function ( i.e. because the second order difference is constant ).

Venkata Karthik Bandaru - 5 years, 5 months ago

Log in to reply

Indeed @Karthik Venkata :)

Rohit Ner - 5 years, 5 months ago

Log in to reply

Better edit the solution to include this :).

Venkata Karthik Bandaru - 5 years, 5 months ago

The last numbers of each row are triangular number , i.e., n ( n + 1 ) 2 \frac{n(n+1)}{2} where n n is the number of row. So I consider this more easier to interpret the first and last numbers of 4 5 t h 45^{th} row.

Akshat Sharda - 5 years, 5 months ago

Log in to reply

i solved it using this observation

Dev Sharma - 5 years, 5 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...