law of tangents
Assume that a, b, c represent the lengths of the three sides of a triangle, and A, B, C represent the angles opposite these three sides. Then the law of tangents states that

symbol space(a-b)/(a+b) = tan[(1/2)(A-B)]/tan[(1/2)(A+B)]
symbol space(b-c)/(b+c) = tan[(1/2)(B-C)]/tan[(1/2)(B+C)]
symbol space(c-a)/(c+a) = tan[(1/2)(C-A)]/tan[(1/2)(C+A)]


 
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