[ATTACH=full]3855[/ATTACH]
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)