The derivative is the limit of the increment ratio of a function. Increment ratio is the ratio of function value increment to independent variable increment. When the function is in a neighborhood of point xo, the ratio of the increment of function value △y=f(x)-f(xo) to the increment of self-complex △x=x-xo △y/△x, and the limit lim△y/△x exists, we say that the function can be found at xo. The function f(x) is differentiable in the domain of definition, and f'(x) is called derivative function, which is called derivative for short.