Which of the following is a key element in the proof of the law of cosines?
a. the triangle inequality theorem
b. angle a+ angle b+ angle=180
c. a=1/2b*h
d. d=square root of (x^2+x^1)+(y^2-y^1)^2

The best and most correct answer among the choices provided by the question is the fourth choice "d=square root of (x^2+x^1)+(y^2-y^1)^2"
The law of cosines for calculating one side of a triangle when the angle opposite and the other two sides are known. Can be used in conjunction with the law of sines to find all sides and angles. 

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For being nice i appreciate it but abc