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For example in y=1/x, variable y and x are inversely proportional.

If y and x have a different relation than inversely proportional, but still y decreases as x increases and vice versa, how do you describe their relation? Thanks!

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    Drop the "proportional" and just refer to it as an inverse relationship.'
    – Jim
    Commented Mar 6, 2014 at 4:15

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Inversely proportional doesn't only describe the direct 1/x case, but the general case where y decreases when x increases. So even if you had a formula like:

y = 12 + 57/(2x + 5)

or

y = 1/x^2

You would still describe the relationship between y and x as inversely proportional.

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    A mathematically proportional relationship requires that the change be related by a constant multiplier, not just moving in opposite or the same directions.
    – ColleenV
    Commented Nov 6, 2014 at 21:48
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"Monotonically decreasing" describes a situation where (as x goes up) y either stays the same or goes down. For "continuous functions", this is the same as saying that dy/dx <= 0.

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