Equivalent formula

Equivalent formula: e x- 1-x (x → 0). There are two propositions, P and Q. If the conclusion that Q can be established on the condition of P, it is said that P is a sufficient condition of Q; If p can be established by q, then p is a necessary condition for q; If p and q can be deduced from each other, it is said that p is a necessary and sufficient condition of q, which is also called equivalence of p and q.

If the relation R is reflexive, symmetric and transitive in the set A, it is called an equivalent relation on A. The so-called relation R is a subset of the cartesian product A× A.

The two elements X and Y in A have a relation R. If (X, Y) ∈ R, we often abbreviate it as xRy.

Reflexive: If any X belongs to A, then X is related to itself, that is, xRx；;

Symmetry: Any X and Y belong to A. If X and Y have a relationship R, that is, xRy, then Y and X also have a relationship R, that is, yRx；;

Transfer: any x, y and z belong to a, if xRy and yRz, then xRz.

If x and y have equivalence relation r, they are said to be equivalent, sometimes called equivalence.