The reason why y=0 is substituted into the derivative of x first is because y = a x (a > 0, a≠ 1), the domain is (-∞,+∞), the range is (0,+∞), and a>0 is a strictly monotonically increasing function (i.e. when x2 >; When x 1, y2 >; Y 1), 0<a< 1 is a strictly simple decreasing function.

For any A, the image passes through the point (0, 1). Note that the graphs of y = a x and y=log(x) are symmetrical about y axis. Logarithms with the base of 10 are called ordinary logarithms and abbreviated as lgx. Logarithm based on e, that is, natural logarithm, is widely used in science and technology, and is recorded as lnx.

situation analysis

But in many practical problems, it is often necessary to study the relationship between the dependent variable and several independent variables, that is, the value of the dependent variable depends on several independent variables. For example, the market demand of a commodity is not only related to its market price.

It is related to the income of consumers and the price of other substitutes for this commodity, that is, there are not one but many factors that determine the demand for this commodity. To study this kind of problems comprehensively, it is necessary to introduce the concept of multivariate function.