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Equivalence of inequalities. Systems and unions of inequalities

Multiplying by an expression
The inequality f(x)<g(x)f(x)<g(x) on a set MM, after multiplying both sides by h(x)>0h(x)>0 (on MM), keeps the inequality sign; if h(x)<0h(x)<0 — the sign flips to the opposite. If hh changes sign on MM, split the domain into intervals where the sign is fixed and solve piecewise.
When h(x)=0h(x)=0, you cannot multiply without a separate analysis of those points
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