Proof ofthe second fundamental theorem
Please, how do we get in the sixth line that f(x) is right the one f(t), also, the sam f function as we have in the lines before?
In other words, how do we KNOW that The SAME f(x) we have in the right side before?
I understand that it is an DERIVATIVE, SURE! "An derivative! " But how do we know it is the same FUNCTION as we have f(t) on the right?