Solve for cos A

[nycmathguy]

 

[MathLover1]

last line is incorrect

a^2-b^2-c^2= (-2bc)*cos(A)
(a^2-b^2-c^2)/(-2bc) = cos(A)
-(a^2-b^2-c^2)/(2bc) = cos(A)
(-a^2+b^2+c^2)/(2bc) = cos(A)

 

[nycmathguy]

last line is incorrect

a^2-b^2-c^2= (-2bc)*cos(A)
(a^2-b^2-c^2)/(-2bc) = cos(A)
-(a^2-b^2-c^2)/(2bc) = cos(A)
(-a^2+b^2+c^2)/(2bc) = cos(A)

I tried. I did my best. Can't get them all right.