Write an equation expressing direct Variation
The circumference of a circle is given by the formula C = 2pr, where r is the radius of the circle. See the figure. Circumference is always a constant multiple of the radius. (C is always found by multiplying r by the constant 2p.) Thus, As the radius increases, the circumference increases. The reverse is also true. As the radius decreases, the circumference decreases. Because of this, the circumference is said to vary directly as the radius. 
Direct Variation
y varies directly as x if there exists some constant k such that y = kx. Also, y is said to be proportional to x.
The number k is called the constant of variation. In direct variation, for k > 0, as the value of x increases, the value of y also increases. Similarly, as x decreases, y decreases.