First, you seem to be using the same letters to signify both corner points and the corresponding angles, which to me is nonstandard. Not a serious issue.
Since you guessed the formula, I would start by checking that it works in various cases, and in particular checking that it gives the same outcome as known formulae for particular quadrilaterals; that is, that you can derive this formula from other known formulae and the other way around in the cases where both apply. Especially the latter activity is likely to lead to more insight on the formulae.
If everything points out to the formula working, you should try to prove it. Study how the formulae for the area of other geometric shapes is proven to learn more about how this can be done, if in doubt.
Good luck!
tommi
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2026-05-20T11:58:06Z (4 months ago)