Question 162639
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It can't be joint, for there has to be more than two variables
to have joint variation.

To tell whether it is direct or inverse:

Substitute any two different values for X and calculate the two 
different values of Y for each.

If when X INcreases, Y also INcreases, and
when X DEcreases, Y also DEcreases then the variation is DIRECT.
(they go up or down in the SAME direction),

If when X INcreases, Y DEcreases, and
when X DEcreases, Y INcreases then the variation is INVERSE.
(they go up and down in OPPOSITE directions),

{{{1/3}}}{{{Y=X}}}

Substitute Y = 3:

{{{1/3}}}{{{(3)=X}}}

{{{1/cross(3)}}}{{{cross(3)=X}}}

{{{X=1}}}

Substitute Y = 6:

{{{1/3}}}{{{(6)=X}}}
 
  {{{2}}}
{{{1/cross(3)}}}{{{cross(6)=X}}}

{{{X=2}}}

So when X INcreased from 3 to 6, Y also INcreased
from 1 to 2, so the variation is DIRECT. 
(They both went UP).

----------------------- 

{{{xy=10}}}

Substitute x = 2:

{{{(2)y=10}}}

{{{y=5}}}

Substitute x = 5:

{{{(5)y=10}}}
 
{{{y=2}}}

So when X INcreased from 2 to 5, Y DEcreased
from 5 to 2, so the variation is INVERSE.
(One went up and the other went down).

Edwin</pre>