Given any triangle ABC with corresponding side lengths a, b, c the law of cosines states:? Can someone explain to me how I get this

As per convention:

- the angle at A is named A, the angle at B is named B, and the angle at C is named C

- the side of leght a is the side opposed to A, the side of length b is opposed to B, and the side of length c is opposed to C.

Now just state the law of cosines:

c^2 = a^2 + b^2 - 2ab cos (C)

Also, a^2 = c^2 + b^2 - 2bc cos (A)

And, b^2 = a^2 + c^2 - 2ac cos (B)