Can someone walk me through Direct Linear Variation? (8th grade)

Question

anonymous · Answer

What specifically do you need to know?

anonymous · Answer

Well, my online math curriculum likes to make this really complicated so like this problem:  

Find the constant of variation for the relationship f(x)= 40x.
 
 A.40
 
 B.x
 
 C.20
 
 D.f(x)

anonymous · Answer

A really quick overview would be the following:
Direct linear variation is a result of the variable being multiplied or divided by a constant. Addition or subtraction will not work. For example:
y=2x where x is the variable and 2 is the constant. When drawn on a graph, it will be a straight line and will pass through point (0,0)

In this case the constant will be 40. This is because, no matter what you change x to, y will always be a multiple of 40, hence it is a constant

anonymous · Answer

Examples of direct linear variation would be:
y=2x, f(x)=20x, f(y)=-3y where the number (including the negative) is the constant
Examples of non direct linear variation would be:
y=9/x, y=x^2, y=2+x

anonymous · Answer

Ahhh!!! Thanks it makes sense now! Fan and medal. :D