I have a missunderstanding of wha's going while mulilpying my equation/formula with variable or divide by variable, I'm now to trying to troll or kidding, I want to learn why it's right , so PLEASE bear me and my ears now are really want to learn, no more nothing else.

I've a struggle when my teacher do like this :

there's a formula := I = U /R Im totally understand this, now my problem is when my teacher write I=(U/R) * U/U is the same as I=U/R and he just multiply by 1, what's confusing me that U might be zero, so he didn't mention that case and also other teachers just multiplying/dividing a formula with variables without clarifying that this variables can't be zero ..

my question is , is it right to multiply U/U on formula I=U/R without saying that U can't be zero? because if it's zero then we cant do U/U ... must I mention once I muliply and divide by variable that this variable can't be zero? about all teachers conventionally forgetting from putting a note that the variable we multiply and divide by it can't be zero .. so is it right to not sign that note while multiplying and dividing with the same variable?

I'm confusing why we didn't mention while multiplying and dividing with same variable that this variable can't be zero .. otherwise we can't do U/U because U might be zero (U is variable)

thanks