We write the elastic function of y=f(x) as Ey/Ex=(△y/y)/(△x/x).

When △x infinitely approaches 0, the limit is needed to find elasticity.

It can represent the elasticity of any point x, which refers to point elasticity here.

If you don't need to ask for a limit, the direct delta is actually a difference. This is the elasticity of the arc, but don't worry so much. You just need to remember that the formula is simplified to the final result and use it directly. After simplification, ey/ex = x f' (x)/y, which is the derivative of y multiplied by x in Chinese.

This is the meaning of flexibility.

Then apply it to economics. For example, the price elasticity of demand, its independent variable is P, which is X in the mathematical formula, and the dependent variable is Q, which is Y in the mathematical formula. Note that the negative sign is sometimes added in economics to highlight the ups and downs, so the price point elasticity of demand is sought.

That is ed =-q' (p) p/q) p/q.