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Why do you have to reverse inequality signs when you divide or multiply both sides of the inequality by a negative number?

Please explain via graphs and/or algebra

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Robert J. | Certified High School AP Calculus and Physics TeacherCertified High School AP Calculus and Ph...
4.6 4.6 (13 lesson ratings) (13)
-2 < -1
Multiply both sides by -1,
2 > 1
Reason: When you multiply both sides by -1, the order on number line from left to right of the numbers reversed.
a ... b becomes -b ... -a.