$3-$

A parallelogram is intersected by two rows of lines parallel to its sides; each row consists of $m$ lines.

How many parallelograms can be identified in the resulting grid

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To define a parallelogram, you need to choose two lines from each row of parallel lines. Note that in the row $m+2$ lines: the sides of the original parallelogram are added.

## Answer

$\left[\frac{(m+1)(m+2)}{2}\right]^{2}$