Puzzle – How many rectangles on a chess board? – Solutions
April 21, 2007 pm30 10:07 pm
This is the place for solutions to the rectangles on the chess board problem. The original problem is here.
This is the place for solutions to the rectangles on the chess board problem. The original problem is here.
A rectangle on a chess board is bounded by two vertical grid lines and two horizontal grid lines. Since a chess board has nine vertical and nine horizontal grid lines, the number of rectangles is C(9,2) * C(9,2) = 1296.
ACTUALLY THERE ARE 8 HORIZONTAL AND VERTICAL LINE
count it, there are 9, since there are 8 boxes
soumya, you are very stupid
allah has spoken!
u are very very stupid
soumya i dint know u r so dumb.
you lack basic knowledge in mathematics.
stupid soumya!!
a** h**** soumya
hehehe bas naam hi kaafi hai
be quite son of b****
Seriously? COUNT THEM!!!!
u r right
David has given the most easy and effective solution, what for all these stupid and confusing discussion……………….
hi dosto
Lovely answer David! The problem puzzled me for a long time.
thanx david!
Also, an alternative soln is summation of cubes from 1 to 8 ie (1+8+27+64+125+216+343+512 =1296)
-Jyoti
no there is not 1296 rectangle there is 1296-204=1092 rectangle
N3-N2 where n=1 2 3……8
1+8+27+64+125+216+343+512-(1+4+9+16+25+36+49+64)=1092
I didn’t understand your logic of summation. Why did you do this?
You’re right, adding the cube numbers. :)
thanx david guetta for the answer
great david !!!!
How do you work out c(9*2) ?
how do you do c(9^2) ?
c(9,2) would be combinations of nine things, taken two at a time. (9 x 8) / (1 x 2) = 36. How do you get it? There are 9 lines (dividing 8 rows). Take any two of them…
Very cool – a brand new approach for me. I have a few others, but this puzzle doesn’t seem to have caught my usual puzzlers’ imaginations. I was waiting for more response.
You can do it the same way as the square problem. The nth bottommost, mth rightmost square is the northwest corner of mn rectangles. The nth row has northwest corners of (1 + … + 8)n = 36n rectangles, so in total there are 36(1 + … + 8 ) = 36^2 = 1296 rectangles.
And in general, on an m*n board there are mn(m+1)(n+1)/4 squares, the same answer you’d get by generalizing David’s approach but by a different method.
My first solution was by dimensions. Consider a 3×3 chessboard.
There are 9 1×1′s, 6 2×1′s, 3 3×1′s
There are 6 1×2′s, 4 2×2′s, 2 3×2′s
There are 3 1×3′s, 2 2×3′s, 1 3×3
Trotting out the LaTeX, that’s
, which equals
, which can be factored as
From there we reach
, or,
as you’ve generalized for any m x n rectangle,
soumya…..u r really really stupid….
n jd2718 ur very very very genious………….
wt abt triangel gerneralised formula..?
Incidentally, this generalizes to n-dimensional boards. A board of size m1*m2*…*m(n) has m1(m1+1)…m(n)(m(n)+1)/2^n n-dimensional rectangular polytopes.
204 squares
Why are ther 204 squares in a chess board?
Because we count all the different sized squares, not just the little ones.
i make it 1365
there are 1296 rectangle in an chess board
i worked out a way an i got 1278
there are 1296 different rectangles in a chessboard..
how many squares and rectangles aee in a chess board…………………………………………………………………………………………………………………………………………………………………………
1296 rectangles + 204 squares
The easiest way to work this out, is to see a patern. So, see how many sqaures and rectangles there are in a 1 by 1 grid and add up total. Answer is 1. do that for 2,3,4,5,6,7 and 8. It may take a while but you will get the answer in the end.
too slow…for my homework, i get a third of a page to explain it
In a standard chessboard, you have 9 horizontal and 9 vertical lines. Choose any two horizontal and any two vertical lines to form a rectangle. Both of them can be chosen in 9c2 ways. So the total number of rectangles is 9c2 * 9c2, one for horiznotal, and one for vertical. Simple
can u explain the same for square…???
That’s gorgeous, thank you.
And, it generalizes eg how many boxes in a Rubik’s cube? (3 x 3 x 3)
dumm puzle
How many different rectangles which are not squares can be found on a chess board? ( a chess board has 8 rows and 8 columns of squares. a rectangle on the board is a collection of squares that form a rectangular piece of the whole board. Two rectangles are different if they have different sets of squares. A rectangle with an unequal number of rows and columns is not square.)
Hi hema, your question is part of the current CUNY math challenge – it wouldn’t be fair for me to answer.
I started from a 1*1 chess board and worked my way up to 1 4*4 chess board before I spotted a pattern.
1*1 = 1 square ‘a’
2*2 = 9 squares ‘b’
3*3 = 36 squares ‘c’
4*4 = 100 sqauers ‘d’
Let us call the number that is beign squared ‘n’.
I saw that the differences between n each time was going up by 1. For example:
number of squares on ‘a’ = 1
= 1*1
number of squares on ‘b’ = 9
= 3*3
number of squares on ‘c’ = 36
= 6*6
number of squares on ‘d’ = 100
= 10*10
The sequence is 1, 3, 6, 10 and therefore the differences are 2,3,4, and so I carried on with the sequence by adding 1 to the differences until I worked out that the total numbrer of rectangles on a chess board must be:
28*28 = 784
The number was lower than I expected, but I have yet to test out the hypothesis.
1296
there are 1296 rec. on chess board
there r 1296 rec. on chess board
and there are 204 squares
YAA Adding this to my bookmarks. Thank You
YAA Adding this to my bookmarks. Thank You
there are thousands of rectangles in a chess boards…
and 1296 is the number of squares..
but if u concentrate the number of rectangles…then it wud b lot more..
No, Ashish. There are 204 sqaures and 1092 rectangles and you add them together to get 1296.
there is 1…the board itself
Absolutely. But there are others as well.
What does 9c2 mean? Im really confused and trying figure this problem out but it is hard not understanding what everything means.. Can someone help, please!
9C2 is combinations of 9 things taken 2 at a time. For example, how many ways could you choose two books out of nine possible? We also write C(9,2) or or
there are 1296 – 204 = 1092 rectangles..
Finally the Answer I have been looking for ..
Out of those 9c2 * 9c2 quadrilaterals, some are squares…So no.of rectangles = no.of quadrilaterals – no.of Squares
1296 rectangles in a chess board
Hey pleas say hw its 1092 rectangles…explain
all square are rectangles but all rectangles are not square so the answer is not 1092 it is 1926
there is a nice solution to the how many squares or rectangles are on a chessboard problem here
http://puzzles.nigelcoldwell.co.uk/twentyseven.htm
with diagrams
hello champions
i think dat problem is related to combination. actually the wright answer is 1296.
thanks david u r so brilliant.
hi stop rude shutt your bitch fat gob
there are 1926
I figured it out in grade 3, honestly. I did it the long way but found a few patterns, and I got 1296 or 32 squared. I am very proud. :)
I found patterns when I figured it out in grade 3 so I didn’t take, like two years. I am NOT lying.
salo bakwas kr rhe ho ans kyo ni dete
288
64 squares
512 rectangles
1296
9C2 *9C2
1296
I’m stuck and this didn’t help at all
there is 1092 rectangles in chess board
and there is 204 square in chess board
respect sirs, i got there are 1296 rectangles on a chess board. but i think it is 1 less means 1295. can you please give me the reply? my email id= mail4sajee@gmail.com
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