7. Which is the direct linear variation equation for the relationship?

y varies directly with x and y = 12 when x = 4.

(Points : 4)
y = 2x + 4
y = 3x
y = x + 8
y = x – 8

Question

7. Which is the direct linear variation equation for the relationship?

y varies directly with x and y = 12 when x = 4.
 
(Points : 4)
       y = 2x + 4
       y = 3x
       y = x + 8
       y = x – 8

anonymous · Answer

y varies directly with x is written as:
$$y \alpha x$$
To make it into an equation we introduce the proportionality constant, k, 
y=kx
So at x=4 and y=12, we substitute into the equation to get
12=k*4
ie k=12/4=3
So the answer is:
y=3x.