Direct Variation

Is y = -8x a direct variation?

YES! y = -8x passes through (0,0).

k

k is the constant of variation. (the same as slope)

Write a direct variation equation for the following: y = 3 when x = 7.

y = kx 3 = 7k Solve for k. Divide both sides by 7. k = 3/7. Write your equation. Plug in 3/7 for k. So y = 3/7x.

Write a direct variation equation for the following: x = 6 when y is 4.

y = kx 4 = 6k Solve for k. Divide both sides by 6. So k = 4/6. now plug this into y=kx. So y = 4/6x.

Direct Variation

y = kx ; look for phrases like "direct variation" or "varies directly". A direct variation equation ALWAYS passes through (0,0)--the origin.

Is y = 3x - 7 a direct variation equation?

ABSOLUTELY NOT! This equation will not pass throught the origin (0,0). ALL direct variation equations pass through (0,0).