Direct Variation

Question

Which of the following is NOT an example of direct variation?

A) y =10x

B) y = x/10

C) y = 100x

D) y/x = 1/100

I understand direct variation as k=y/x or y=kx, but am not sure how to put these equations into an equation that includes k.

I also know the answer is A, but need clarification with how it was derived. Looking at the other choices:

A) y=10/x

B) y=x/10 I am told by my book that this becomes k=1/10. Where does k come from? Also,

C) y=100x becomes k=100. How do you arrive at k?

D) y/x=1/100 becomes y=x/100 (cross multiplication makes sense, but not how it's eliminated)

Can anyone make sense of this for me?

Answer 1

y=10/x is not direct variation, as it's an inverse relation. direct is y=kx, inverse is y=k/x

all the other answers are direct and linear straight line relations.

y=10/x is inverse and a hyperbola, not a line