Step-by-step explanation:
The Law of Cosines specifies the relationship between the three sides of a triangle and any one of the angles. If the sides are designated a, b, and c, and the angle opposite side c is C, then it tells you ...
c² = a² + b² -2ab·cos(C)
This relationship can be used to find any and all angles, given the three sides of a triangle. Or, having found one angle using the Law of Cosines, the others can be found using the Law of Sines:
sin(A)/a = sin(B)/b = sin(C)/c
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Typically, inverse functions are required. That is, from the Law of Cosines, ...
C = arccos((a² +b² -c²)/(2ab))
And from the Law of Sines, ...
A = arcsin(a/c·sin(C))
B = arcsin(b/c·sin(C))
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Note on solving triangles
It often works best to make use of exact values where possible. It is also a good idea to start with the longest side/largest angle. Of course, once you have two angles the other can be found as the supplement of their sum.