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Q.
The number of rectangles which we can form on a chess board is
Permutations and Combinations
Solution:
There ate 9 horizontal and 9 vertical lines on a chess board. To from a rectangle we require two horizontal and two vertical lines. Thus the no. of rectangles on the chess board
$=\,{}^{9}C_{2} \times \,{}^{9}C_{2}=36 \times 36$
$=1296$.