| Direct Variation | 
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 When a value "y" increases as a value "x" increases, direct variation is said to be present.  | 
Below, "x" and "y" vary directly. As one increases, the other increases.
 
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Figure 1. | 
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| Inverse Variation | 
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 "y" varies inversely with "x" if there is a nonzero constant such that xy = k.  | 
Here, "rate" and "time" vary inversely at a constant rate. As one increases, the other decreases:
 
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Figure 2. | 
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