sinA / a=sinB / b=sinC / c=m; m>0.
sin^2A=sin^2B+sin^2C+sinBsinC,
(am)^2=(bm)^2+(cm)^2+(bm)(cm),
a^2=b^2+c^+bc=b^2+c^-2cosA * (bc);
cosA=-1/2;
A=120