help!

If x is directly proportional to y, we say x\(= ky\) (k is a constant)
If x is inversely proportional to z, we say \(x = \frac{k}{z}\) (k is a constant)
Now, in case of joint variation i.e. when both these relations hold, we say \(x = \frac{ky}{z}\)
This is the concept of Joint Variation.

See if this makes sense.
Say, \(x = \frac{ky}{z}\)
Here, when z and k are constants, x is directly proportional to y.
When y and k are constants, x is inversely proportional to z.
Hence, both relations are satisfied.
That is the reason we say, "Keeping all else constant, x is directly proportional to y." What we mean is that if x varies with any other variables, keep them all constant. Then relation between x and y is of direct variation.

Does indirectly proportional means inversely proportional??

Here it says X is indirectly proportional to Z...what does that mean??