B~ can be deduced from A, and A ~ can be deduced from B, then A is the necessary and sufficient condition of B.

A is a necessary and sufficient condition for B, if B can be deduced from A, but not from B..

A is a necessary and sufficient condition for B. If B cannot be deduced from A, A can be deduced from B..

If b~ cannot be deduced from A ~ and A ~ cannot be deduced from b, then a is a necessary and sufficient condition for B.

Simply put, a condition can lead to a conclusion, but this condition cannot be led from the conclusion. This condition is a sufficient condition.

If we can infer from the conclusion,

Conditions, but can't draw a conclusion from the conditions. This condition is necessary.

If we can not only deduce conditions from the conclusion, but also have conditions.

Draw a conclusion. This condition is necessary and sufficient.

For example, go shopping in the mall.

Payment is a must.

You can't take things without paying.

But money alone is not enough.

So the necessary condition is that you must pay the money (otherwise it is not negotiable)

The sufficient condition is that you have enough money (enough money to buy)

That is to say!

Necessary conditions are necessary.

But even if it is all, it may not be successful (money may not buy things)

But money is definitely not enough)

If all the sufficient conditions are met.

That's a success.

Take it.

thank you