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When two variable quantities have a constant (unchanged) ratio, their relationship is called a direct variation. It is said that one variable "varies directly" as the other.

The formula for direct variation is y = kx, where k is the constant of variation. "y varies directly as x"

For example:
The weekly salary of a girl earns, S, varies directly as the number of hours, H, which she works. Express this relation as a formula. 

The answer is: S = hk or \frac{s}{h} = k[/tex]
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Example of Direct Variation in real life application:

1)  The voltage (V) of an electric circuit varies directly as the current (I), where R (resistance is constant)
      V = R × I   or  V=RI

2)  Hooke's law in physics states that the extension (S) of a spring varies directly as the applied force (F), where k is the constant of variation
     S = kF

3)  The interest (I) payable on a given loan varies directly as the time (t) for which the loan is made, where k is the constant of variation.
     I = kt