# Thread:

1. ## another variation

find the required value by setting up the general equation and then evaluating.

find y when x=10 if y varies directly as x and y=20 when x=8

thanks!

2. Originally Posted by mamajen
find the required value by setting up the general equation and then evaluating.

find y when x=10 if y varies directly as x and y=20 when x=8

thanks!
When y varies directly as x, we have the equation $y = kx$. The first thing to do is solve for k. Substituting in y = 20 and x = 8 we have $20 = 8k$, or $k = \frac{20}{8} = \frac{5}{2}$.

Given that $y = \frac{5}{2}x$, can you determine y when x = 10?

3. yes i can get it from there. thanks. I was not thinking to find k first!

4. Icemanfan, hope you don't mind me just adding something.

mamajen, In the other question you asked, I said what "proportional" mean and told you about putting the k there and then working out what k was. Well, "varies directly as" just means "is proportional to", so the two questions are quite similar.

5. thanks iceman. i remembered that part but didn't know where to put all the equal to #'s. then when I saw he used the 2nd y= to find k I knew where I was going
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