This is a free lesson from our course in Trigonometry This lesson defines and deals with the basic concepts of laws of sines and cosines and their applications. The Sine Law : When you need to work with triangles that have no right angles, you can use the Sine Law. This law can be used when you know 3 pieces of information about the triangle and one known side is directly across from a known angle. If we donít have this last property, we need to use the Cosine Law. The Cosine Law : As explained above, sometimes we have situations where the sine law will not work. This happens when we do not have a side which corresponds to an angle in the triangle. In such cases, we use the Cosine Law. (More text below video...)
Other useful lessons:
 Using the law of Sines and Cosine Formulas The Ambiguous Case Area of a Triangle
(Continued from above) For a triangle ABC with sides a, b, and c and opposite angles A, B and C, the law of sines is:
sin A / a = sin B / b = sin C / c
For the same sides and angles, laws of cosines are:
c2 = a2 + b2 - 2ab cos C.
b2 = a2 + c2 - 2ac cos B.
a2 = b2 + c2 - 2bc cos A.
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