[(a^2 + c^2 -b^2) / （2ac）] / [(a^2 + b^2 - c^2) / (2ab)] = -b / (2a + c)2a (a^2 + c^2 - b^2) + c(a^2 + c^2 - b^2) + c(a^2 + b^2 - c^2) = 02a (a^2 + c^2 - b^2 + ac) = 0 所以 有cosB = -1/2