Law of Cosines 5.2

Question

For the triangle ABC, which of these is not a Law of Cosines?

 

(a) a²=b²+c²-2bc cos A

 

(b) b²=c²+a²- 2ac cos B

 

(c) c²=a²+b²-2ab cos C

 

(d) a²=b²+c²-2ab cos A

 

Please show step by step work to solution and provide final solution. Please and Thank You!

Answer 1

Option d) does not conform to Law of Cosines usage because the coefficient of cos A must be -2bc.