# If Y varies inversely with the square of x. When x is 5, y is 4/5, how do you find value of y, when x equals 8?

Apr 22, 2016

Set up the described equation, solve it for the proportionality constant, and then use your completed equation to calculate the new value.

#### Explanation:

First, set up the described equation.
$y = a \cdot \frac{1}{x} ^ 2$
Then, given x = 5 and y = 4/5 we can solve for the proportionality constant of the equation.
 4/5 = a * 1/(5)^2 ; a = 4/5 * (5)^2 ; a = 4/5 *25 = 20
SO, our equation is $y = \frac{20}{x} ^ 2$

$w h e n x = 8 , y = \frac{20}{8} ^ 2$ ; y = 20/64 = 0.3125