SOLUTION: I can't figure this out for the life of me...HELP!!! This has to do with direct and inverse variation. The problem is: x varies directly as the square of s and inversely as t.

Algebra ->  Coordinate Systems and Linear Equations  -> Linear Equations and Systems Word Problems -> SOLUTION: I can't figure this out for the life of me...HELP!!! This has to do with direct and inverse variation. The problem is: x varies directly as the square of s and inversely as t.       Log On


   



Question 125838: I can't figure this out for the life of me...HELP!!! This has to do with direct and inverse variation.
The problem is: x varies directly as the square of s and inversely as t. How does x change when s is doubled? When both s and t are doubled?
Please help me.

Found 2 solutions by Earlsdon, josmiceli:
Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
You can express "x varies as the square of s and inversely as t" like this:
x+=+ks%5E2%2Ft k is the constant of variation.
To see how x changes when s is doubled (2s), simply substitute 2s for s in the above formula and simplify.
x+=+k%282s%29%5E2%2Ft
x+=+k%284s%5E2%29%2Ft
x+=+4%28ks%5E2%2Ft%29 but, as you can see, the factor in parentheses (ks%5E2%2Ft) is the original x, so...
x+=+4x or, in words, x is quadrupled when s is doubled.
Now, when both s and t are doubled, substitute 2s and 2t for s and t respectively in the first formula:
x+=+ks%5E2%2Ft
x+=+k%282s%29%5E2%2F2t Simplify.
x+=+k%284s%5E2%29%2F2t Cancel 2's in the top and bottom.
x+=+2%28ks%5E2%2Ft%29 or...
+x+=+2x so you see that x is doubled when both s and t are doubled.

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
You can write an equation using what is called "the constant
of proportionality k" Since there are 2 equations, I'll
use 2 constants k%5B1%5D and k%5B2%5D
x+=+k%5B1%5D%2As%5E2
x+=+k%5B2%5D%2A%281%2Ft%29
------------------
The constants have to stay constant, so
k%5B1%5D+=+x+%2F+s%5E2
If I double s
k%5B1%5D+=+4x+%2F+%282s%29%5E2
k%5B1%5D+=+4x+%2F+4s%5E2
x must be multiplied by 4 so that k%5B1%5D gets
multipied by 4%2F4 and does not change
-------------------
If both s and t are doubled:
x+=+k%5B2%5D%2A%281%2Ft%29
k%5B2%5D+=+x+%2F+%281%2Ft%29
k%5B2%5D+=+t%2Ax
k%5B2%5D+=+%282t%29%2A%28x%2F2%29
You can see that x must be multipied by 1%2F2 so that k%5B2%5D
will stay the same. So x gets multipied by 4 and 1%2F2
or 4%2A%281%2F2%29+=+2 x is doubled. answer