是不是 (a+b)/(b+c) = c/(a-b)?按这个计算由等式有a²-b² = bc+c²a² = b²+c²+bc由三角形余弦公式：a² = b²+c²-2bc * cosA所以有：-2cosA = 1, cosA = -1/2所以A = arccos(-1/2) = 120°
