A nice,unconventional Algebra question.

Level pending

{ Row A : 1 , 2 , 3...9 Row B : 2 , 3 , 4...10 Row C : 3 , 4 , 5...11 . . . . Row I : 9 , 10 , 11...17 Row J : 10 , 11 , 12...18 \begin{cases}\text{Row A}:\ \ 1,2,3...9\\ \text{Row B}:\ \ 2,3,4...10\\ \text{Row C}:\ \ 3,4,5...11\\ .\\.\\.\\.\\ \text{Row I}:\ \ 9,10,11...17\\ \text{Row J}:\ \ 10,11,12...18\end{cases} .Find the sum of all the numbers in all the rows.


The answer is 855.

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.

3 solutions

Sum of Row A \text{Row A} is 9 2 ( 1 + 9 ) = 45 \frac { 9 }{ 2 } \left( 1+9 \right) =45 .

Sum of Row B \text{Row B} is 45 + 9 ( 1 ) = 54 45+9(1)=54 , sum of Row C \text{Row C} is 54 + 9 ( 1 ) = 63 54+9(1)=63 , and so on.

The sums form an AP with a 1 = 54 { a }_{ 1 }=54 and a 10 = 126 { a }_{ 10 }=126 .

Hence, 10 2 ( 45 + 126 ) = 855 \frac { 10 }{ 2 } \left( 45+126 \right) =855 .

You can look at the rows diagonally :)

Chew-Seong Cheong
Apr 26, 2015

The sum of all numbers of Row A to Row J is given by:

S = k = 1 10 9 ( k + k + 8 ) 2 = k = 1 10 9 ( k + 4 ) = 9 k = 1 10 k + 9 k = 1 10 4 = 9 ( 10 ) ( 11 ) 2 + 9 ( 10 ) ( 4 ) = 495 + 360 = 855 \begin{aligned} S & = \sum_{k=1}^{10} {\dfrac{9(k+k+8)}{2}} = \sum_{k=1}^{10} {9(k+4)} = 9\sum_{k=1}^{10} {k} + 9\sum_{k=1}^{10} {4} \\ & = \dfrac {9(10)(11)}{2} + 9(10)(4) = 495 + 360 = \boxed{855} \end{aligned}

@Karthik Venkata ,good to have you back man!where were you all this time?how did you find this new problem?

Adarsh Kumar - 6 years, 1 month ago

Log in to reply

Hi, I used AP sum formula :) ! I was actually on a holiday bro .

Venkata Karthik Bandaru - 6 years, 1 month ago

Log in to reply

you mean on all the rows,individually?

Adarsh Kumar - 6 years, 1 month ago

Log in to reply

@Adarsh Kumar All rows sum have same number of terms and common difference, and the only ones different are the first terms of the rows..

Venkata Karthik Bandaru - 6 years, 1 month ago

Log in to reply

@Venkata Karthik Bandaru the different ones are the first and the last terms of the rows.

Adarsh Kumar - 6 years, 1 month ago

Log in to reply

@Adarsh Kumar Hey I do not mean that, I am talking only about variables in.AP sum formula. Better let's chat on WhatsApp

Venkata Karthik Bandaru - 6 years, 1 month ago

Log in to reply

@Venkata Karthik Bandaru oh!ok bye!

Adarsh Kumar - 6 years, 1 month ago

Log in to reply

@Adarsh Kumar Bye bye bro !

Venkata Karthik Bandaru - 6 years, 1 month ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...