To prove that a function is bounded, must its upper and lower bounds be opposite to each other

no
If there are two a and B, and a ≤ f (x) ≤ B for all x ∈ DF, then the function y = f (x) is said to be bounded in DF, otherwise it is unbounded
Prove that a, B exist on the line, not the opposite number
For example, y = SiNx + 1 is bounded, upper bound ≥ 2, lower bound ≤ 0

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