F represents a mapping relationship, and X can be regarded as a mapping set.

For example, x={ 1, 2,3} can be regarded as (+1) through the relation f, and then x is mapped to {2,3,4}, that is, 1, 2+ 1, 3+65438+.

Later, people regarded the set of X as a domain, the relation F as a mapping about X, and the mapping set as a range of values.

That is, the function f(x) described later.

The partial derivative of the function f(x) is sometimes recorded as f(x) and sometimes as f' or df/dx.

Fx=lim=[f(x)-f(x0)]/(x-x0), and x-x0 is close to zero.