If two quantities are directly proportional then their quotient is an invariant(it does not change, it is constant). Further, if we have another pair of two quantities that are directly proportional then their quotient is also an invariant. Moreover, if you multiple these two quotients, you end up with a third invariant.(adsbygoogle = window.adsbygoogle || []).push({});

But, consider the following example.

Given that x is directly proportional to y and to z and is inversely propotional to w, and that x=4 when (w,y,z)=(6,8,5), what is x when (w,y,z)=(4,10,9)?

Correct Solution:

xw is a contant, x/z is a constant, and x/y is a constant.

Thus, xw/yz is constant. (They just combined the constant terms)

so, xw/yz=(4)(6)/(8)(5)=3/5.

Thus, when (w,y,z)=(4,10,9), we find

x=3yz/5w=27/2.

Wrong Solution a.k.a My Solution:

Since xw is a contant, x/z is a constant, and x/y is a constant. Therefore if we multiply all these constant terms we will get a constant.

Thus, (x^3)w/yz is a constant. The rest is immaterial since I do not end up with 27/2.

I guess there is something wrong with (x^3)w/yz being a constant. Can anybody please explain to me why (x^3)w/yz is not a constant? Hence explain why I do not get 27/2 if my method is followed????

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Direct, and Inverse Proportion; Invariants.

**Physics Forums | Science Articles, Homework Help, Discussion**