Question

Suppose y varies directly with x, and y= 10 when x=-2. What direct variation equation relates x and y? What is the value of y when x=-15. I don't understand how to solve this, can someone explain to me how to get the answer and what it's asking. P.s. it's a practice question so i know the answer just not how to get it.

Answer 1

The statement " y varies directly as x ," means that when x increases,y increases by the same factor. In other words, y and x always have the same ratio:

 = k   

where k is the constant of variation.
We can also express the relationship between x and y as:

y = kx   

where k is the constant of variation.

Since k is constant (the same for every point), we can find k when given any point by dividing the y-coordinate by the x-coordinate. For example, if y varies directly as x , and y = 6 when x = 2 , the constant of variation is k =  = 3 . Thus, the equation describing this direct variation is y = 3x .

Example 1: If y varies directly as x , and x = 12 when y = 9 , what is the equation that describes this direct variation?

k =  =  
y =  x

Example 2: If y varies directly as x , and the constant of variation is k =  , what is y when x = 9 ?

y =  x = (9) = 15

As previously stated, k is constant for every point; i.e., the ratio between the y -coordinate of a point and the x -coordinate of a point is constant. Thus, given any two points (x 1, y 1) and (x 2, y 2) that satisfy the equation,  = k and  = k . Consequently,  =  for any two points that satisfy the equation.

Example 3: If y varies directly as x , and y = 15 when x = 10 , then what is y when x = 6 ?

 =  
 =  
6() = y 
y = 9