Question 1196144
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Angle C is concluded between the sides "a" and "b".


Apply the cosine law to find the third side of the triangle, opposite to the given angle.



On cosine law, see your textbook (or any textbook on Geometry/Trigonometry).



An alternative source are the lessons in this site


&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Trigonometry-basics/Solve-triangles-using-Law-of-Cosines.lesson>Solve triangles using Law of Cosines</A> 


&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Trigonometry-basics/Proof-of-the-Law-of-Cosines-revisited.lesson>Proof of the Law of Cosines revisited</A>