(1) If B can be derived from A and A cannot be derived from B, then A is a necessary and sufficient condition for B (A is included in B).

For example, if a quadrilateral is a rectangle, it is a parallelogram. The opposite is not the case.

therefore

"Quadrilateral is a rectangle" is a necessary and sufficient condition for "Quadrilateral is a parallelogram"

(2) If B cannot be derived from A and A can be derived from B, then A is a necessary and sufficient condition for B (B is included in A).

For example, if the ground is wet, it doesn't necessarily mean it will rain.

So we say, "Rain is a necessary and sufficient condition for wetlands".

(3) If B can be deduced from A and A can be deduced from B, then A is a necessary and sufficient condition for B (A=B).

For example: Condition A He got full marks in the exam: Condition B He did every question correctly.

(4) If B cannot be derived from A and A cannot be derived from B, then A is an insufficient and unnecessary condition of B (A￠B and B￠A).

Example: 0

The reason for this is the following：

If a is a negative number

Then ab < 1 gets b> 1/a,

So this is not enough.

b & lt 1/a

A<0 is ab> 1

So it's not necessary.