kitchen table math, the sequel: another test - U. Wash

## Sunday, March 14, 2010

### another test - U. Wash

ESS 102 Math Assessment

Only 16% could solve for x:

y = x/x-1

farmwifetwo said...

Shouldn't it be "only 16% could solve for Y"?

y=1-1 = 0

ChemProf said...

They mean solving for x as a function of y, and you should read that as y = x/(x-1). This is the kind of fraction that students have fits with, since they want to break up the denominator in creative ways.

rocky said...

This same professor, Cliff Mass, posted this same problem on youtube about three years ago.

He is a professor of Atmospheric Science at the University of Washington, and the video asks people to read wheresthemath.com and to reject the NCTM progressive math standards.

I'm surprised the "cheaters network" hasn't told more people the answers to his little test (psst: 4b is its own inverse!).

Beth said...

OK, I'll try it. I assume "solve for x as a function of y" means "massage the equation until you have x on one side of the eqn, with only ys on the other side."

We start here:

y = x/(x-1)

Multiply both sides by (x-1):

y(x-1) = x

We're not happy yet because we have x on both sides of the eqn. We continue by simplifying:

xy - y = x

Move all x-related stuff to one side:

-y = x - xy

Rewrite rhs:

-y = x(1 - y)

Now divide both sides by (1 - y):

-y/(1-y) = x

et voila!

I'm reviewing math previous to teaching dd, so if I did that wrong, please let me know!

VickyS said...

You're right; you just need to further simplify:

[-y/(1-y)] * (-1/-1) = x

y/(y-1) = x

Catherine Johnson said...

Did I write that wrong?

Probably.

Catherine Johnson said...

hmmm... it says "solve for x"

Anonymous said...

Solve for x works if you put the parentheses in: y=x/(x-1)

LSquared