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Number of squares (Tomato subjective 59)

Problem: Consider the set of point
S = { (x,y) : x,y are non-negative integers \({\le {n}} \) }.
Find the number of squares that can be formed with vertices belonging to S and sides parallel to the axes.
Solution: S = {(x,y) : x,y are non-negative integers \({\le {n}} \) }
We calculate number of squares by calculating number of |x| squares ,& number of squares number of \({{n}* {n}} \) squares.
Now number of |x| squares = number of choosing one pair of lines with difference 1 parallel to x axis & integer distance x number of choosing one pair of lines to y axis with distance 1 & integer distance from y axis = \({{n}*{n}} \) = \({n^2} \)
Similarly number of \({{k}*{k}} \) squares
= \({(n-k+1)^2} \)
So total number of squares
= \({\sum_{k=1}^{n}}{{k}^{2}} \) = \({\frac{n(n+1)(2n+1)}{6}} \)

June 14, 2015
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