Hi guys ! I hope this thread wouldn't be closed because I really struggle that and I need to understand and LEARN!

if I have any equations doesn't matter what it's , for instance f(x) = x^2+6 , I've a problem that I deeply know if I want to move forward in the solution then I must assign into the equation , I mean, I have a case in my problem/question which I must assign into the equation x0 (*specific case* but it's satisfy the equation) , then my question can I assign f(x0)=(x0)^2 + 6 and continue with my solution? what I mean by that I have the equation in general "f(x)=x^2+6 " , in whatever way in my question I arrived to conclusion that x0 in my question must satisfy the equation, then can I assign it into the equation? what's confused me x0 is a specific case that must satisfy the equation .. but the equation is general case .. so how do we assign speicfic x0 into the equation which it's general?

another question, the teacher in the video said if we have X(Y+Z) and we already know that Y is regardless to Z then X(Y+Z) is approximated to X*Z , why it's right? I mean why it's right to disregard Y which it's inside the parentheses .. if it was Y+Z explicitly without any parentheses then I accept the approximation .... what's confusing me why it's allowed to disregard what's inside the parentheses without worrying what's going out of the parentheses !

if I have any equations doesn't matter what it's , for instance f(x) = x^2+6 , I've a problem that I deeply know if I want to move forward in the solution then I must assign into the equation , I mean, I have a case in my problem/question which I must assign into the equation x0 (*specific case* but it's satisfy the equation) , then my question can I assign f(x0)=(x0)^2 + 6 and continue with my solution? what I mean by that I have the equation in general "f(x)=x^2+6 " , in whatever way in my question I arrived to conclusion that x0 in my question must satisfy the equation, then can I assign it into the equation? what's confused me x0 is a specific case that must satisfy the equation .. but the equation is general case .. so how do we assign speicfic x0 into the equation which it's general?

another question, the teacher in the video said if we have X(Y+Z) and we already know that Y is regardless to Z then X(Y+Z) is approximated to X*Z , why it's right? I mean why it's right to disregard Y which it's inside the parentheses .. if it was Y+Z explicitly without any parentheses then I accept the approximation .... what's confusing me why it's allowed to disregard what's inside the parentheses without worrying what's going out of the parentheses !

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