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In trigonometry, Mollweide's formula, sometimes referred to in older texts as Molweide's equations,[1] named after Karl Mollweide, is a relationship between sides and angles in a triangle.[2]

Let a, b, and c be the lengths of the three sides of a triangle. Let α, β, and γ be the measures of the angles opposite those three sides respectively. Mollweide's formula states that

$\frac{a + b}{c} = \frac{\cos\left(\frac{\alpha - \beta}{2}\right)}{\sin\left(\frac{\gamma}{2}\right)}$

and

$\frac{a - b}{c} = \frac{\sin\left(\frac{\alpha - \beta}{2}\right)}{\cos\left(\frac{\gamma}{2}\right)}.$

Each of these identities uses all six parts of the triangle—the three angles and the lengths of the three sides.

Mollweide's formula can be used to check solutions of triangles.[3]

## Notes

1. Ernest Julius Wilczynski, Plane Trigonometry and Applications, Allyn and Bacon, 1914, page 102
2. Michael Sullivan, Trigonometry, Dellen Publishing Company, 1988, page 243.
3. Ernest Julius Wilczynski, Plane Trigonometry and Applications, Allyn and Bacon, 1914, page 105

## References

• H. Arthur De Kleine, "Proof Without Words: Mollweide's Equation", Mathematics Magazine, volume 61, number 5, page 281, December, 1988.