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| | Heron's Formula and Brahmagupta's Generalization |
 | | The actual origin of this formula is somewhat obscure historically, and it may well have been known for centuries prior to Heron. |
 | | Furthermore, it implies that the cosine is given by the well-known formula / a^2 + b^2 - c^2 \ cos(q) = (-----------------) \ 2ab / [By the way, permutations of {a,b,c} = {3,5,7} in equation (2) give the three factorizations 675 = (15)(45) = (9)(75) = (5)(135), which leaves out (1)(675), (3)(225), and (25)(27). |
 | | Incidentally, the formula for the area of an arbitrary quadrilateral is 1 ________________________________________________________ --- /(a+b+c-d)(a+b-c+d)(a-b+c+d)(-a+b+c+d) - 16 abcd cos(q)^2 4 where q is half the sum of two opposite angles. |
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