The great advantage of these three proofs is their universality – they work for acute, right, and obtuse triangles. The theorem states that for cyclic quadrilaterals, the sum of products of opposite sides is equal to the product of the two diagonals:Īfter reduction, we get the final formula: Then, for our quadrilateral ADBC, we can use Ptolemy's theorem, which explains the relation between the four sides and two diagonals. Thus, we can write that BD = EF = AC - 2 × CE = b - 2 × a × cos(γ). CE equals FA.įrom the cosine definition, we can express CE as a × cos(γ). The heights from points B and D split the base AC by E and F, respectively. We also take advantage of that law in many Omnitools, to mention only a few:Īlso, you can combine the law of cosines calculator with the law of sines to solve other problems, for example, finding the side of the triangle, given two of the angles and one side (AAS and ASA).Īnother law of cosines proof that is relatively easy to understand uses Ptolemy's theorem:Īssume we have the triangle ABC drawn in its circumcircle, as in the picture.Ĭonstruct the congruent triangle ADC, where AD = BC and DC = BA The law of cosines is one of the basic laws, and it's widely used for many geometric problems. That's why we've decided to implement SAS and SSS in this tool, but not SSA. Just remember that knowing two sides and an adjacent angle can yield two distinct possible triangles (or one or zero positive solutions, depending on the given data). The third side of a triangle, knowing two sides and an angle opposite to one of them (SSA): The angles of a triangle, knowing all three sides (SSS): The third side of a triangle, knowing two sides and the angle between them (SAS): You can transform these law of cosines formulas to solve some problems of triangulation (solving a triangle).
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