The aim of this paper is to describe the solution (f, g) of the equation
$$\begin{aligned}{}[f(x)-f(y)]g'(\alpha x+(1-\alpha )y)= [g(x)-g(y)]f'(\alpha
x+(1-\alpha )y),\ x,y\in I, \end{aligned}$$ where $$I\subset \mathbb {R}$$ is an
open interval, $$f,g:I\rightarrow \mathbb {R}$$ are differentiable, $$\alpha $$
is a fixed number from (0, 1).... hiện toàn bộ