Hang on, I can barely read this, are you asking to prove that the supremum of is 0 if x > 0?
i need to prove that
i succeded proving that 1 is an upper bound of
now i need to prove that 1 is the lowest upper bound. i tried to assume that there's another number (let called ), that is also an upper bound of but is lower than 1.
but i don't really know how to go forward..
what can i do? i'm so suck in proving things...