Dominoes Covering

There are many ways to cover a chess board completely with 2 × 1 2 \times 1 dominos. In 1961, British physicist M.E.Fisher provedthat it can be done in 12988816 ways.

Now let us cut out two diagonally opposite corner squares. In how many ways can you cover the 62 squares of the mutilated chess board ?

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8 0 1 64

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2 solutions

Biswaroop Roy
Mar 3, 2014

Let us colour the 2 * 1 dominoes into a black and a white pair each.Now we are cutting two diagonally opposite squares. let these two be both black (note that both has to be of same colour.).so now we have 32 white and 30 black squares on the chess board. Obviously its impossible to fill the mutilated chess board,as on placing dominoes,number of black and white squares must be equal.

PROBLEM SOLVING STRATERGIES?

A Former Brilliant Member - 7 years, 3 months ago

is it some kind of rule that both the numbers of dominoes (b/w) be equal ?

Askari Syed - 7 years, 3 months ago

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No its not a RULE. Its a technique called "Colouring Proof Strategy" or something like that.Many sums can be done using this strategy.Well the one I put up was easiest and simplest of them.

Biswaroop Roy - 7 years, 3 months ago

Since dominoes cannot be placed diagonally, they will always cover two squares when placed; two adjacent squares (and by this I mean edges touching, not corners). Given any two adjacent squares, one is black and the other is white, so basically, a given domino will cover 1 black square and 1 white square no matter where it is placed.

Since the chess board is 8x8, we know that opposite corners will be of the same colour (imagine a 2x2 board). Thus, either two white squares are removed or two black squares are removed. Now the number of black and white tiles are not equal (ex. 30 black 32 white). Since a domino must cover a white tile and a black tile whenever it's placed, when the 30 black tiles are accounted for, so will 30 white tiles. This leaves 2 white tiles to be covered, and as stated above, this is impossible to do with a 2x1 domino.

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