240. Three vertices of a parallelogram (in order of traversal) have coordinates $(a, b),(0,0)$ and $(c, d)$. What are the coordinates of the fourth vertex?
$\Delta$ We have already seen that if we count the same number of cells horizontally and vertically, we get an equal and parallel segment. Therefore, we need to count from the point $(c+d)$ to the right by $a$ cells and up by $b$ cells. This will result in the point $(c+a, d+b) . \triangleleft$

Physicists call this operation vector addition: two vectors (solid arrows in the diagram) add up to a third (dashed arrow). If the vectors represent force (direction and magnitude), then two forces applied to the same point can be replaced by their sum

![](https://cdn.mathpix.com/cropped/2024_05_21_90703b5d5e76e3b5cd3dg-074.jpg?height=294&width=352&top_left_y=1252&top_left_x=1366)
(resultant), which is found using the described parallelogram rule.