A cause B

Nyanyanya

When there is a premise saying A cause B, For example.
A.
Premise(1): A Cause B
Premise(2). There is B
Conclusion: There is A
Is this a valid way of reasoning?

B. This is valid reasoning, right?
Premise(1): A Cause B
Premise(2). There is A
Conclusion: There is B

Accounts Playable

No, both examples are not valid.

In argument 1, there could be multiple causes of B. The existence of B doesn't necessarily indicate the existence of A.

Smoking causes cancer.
I have cancer.
Does that by necessity mean that I am a smoker? No; I could have worked on 3 mile island or something and never smoked a day in my life.

In argument 2, this is also not valid. Causality are really measured in degrees of probability, not conditionality. Let's use the same argument:

Smoking causes cancer.
I smoke.
Therefore, I have or will get cancer. This is empirically false because there are people that smoke and never get cancer. You can argue that their likelihood of getting cancer has increased (i.e. the degree of probability) but they won't get cancer out of necessity.

I think the issue you're having is differentiating between conditional logic and causal logic. They are different types of reasoning. Read the "Contrasted with conditionals" section here: https://en.wikipedia.org/wiki/Causality#Contrasted_with_conditionals