12 Given that $y=f(x)$ is a monotonic function defined on $\mathbf{R}$, real numbers $x_{1}<x_{2}, \lambda \neq-1, \alpha=$ $\frac{x_{1}+\lambda x_{2}}{1+\lambda}, \beta=\frac{x_{2}+\lambda x_{1}}{1+\lambda}$, if $\left|f\left(x_{1}\right)-f\left(x_{2}\right)\right|<|f(\alpha)-f(\beta)|$, find the range of values for $\lambda$.
(12) $\lambda<0$