I can't find a definition of it anywhere. How would you find the area of an arbitrary quadrilateral?

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A 'definition' is different than a 'formula'. As for formulas, there are some, including the one given in the answer. One other approach would be to consider two triangles and calculate their areas, or consider the formula $S=d_1 \cdot d_2\cdot \sin \theta /2$ where $d_i$ are the length of the diagonals and $\theta$ is the angle between them. – Beni Bogosel Dec 2 '11 at 21:01
ok. thank you. do you know what am arbitrary quadrilateral exactly is? – tyler w Dec 2 '11 at 21:04
@tylerw - it means any quadrilateral – Victor Dec 2 '11 at 21:07
oh. ok. thank you. – tyler w Dec 2 '11 at 21:09
It is as opposed to special quadrilaterals, like squares, rectangles, rhombuses, parallelograms, and trapezoids. There may be more to be opposed to. – Ross Millikan Dec 3 '11 at 5:49

It's a bit easy to copy formulae from Wikipedia. Right-click on the image, choose "properties", and in the alt-text, the $\LaTeX$ code is often shown. You can copy that and paste here for MathJax to parse. – J. M. Dec 3 '11 at 1:30
@J.M. Many browsers (e.g. Safari and Chrome) do not have that feature. One can “Edit” the page on Wikipedia and copie the $\LaTeX$ from there (replacing [itex]…</math> with $…$, though. – Kevin Reid Dec 3 '11 at 18:36