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
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8 comments:

farmwifetwo said...

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

y=1-1 = 0

March 14, 2010 at 4:11 PM
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.

March 14, 2010 at 4:47 PM
rocky said...

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

http://www.youtube.com/watch?v=ymvSFunUjx0

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!).

March 14, 2010 at 6:13 PM
FedUpMom 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!

March 15, 2010 at 9:17 AM
VickyS said...

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

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

y/(y-1) = x

March 15, 2010 at 6:41 PM
Catherine Johnson said...

Did I write that wrong?

Probably.

March 16, 2010 at 12:35 PM
Catherine Johnson said...

hmmm... it says "solve for x"

March 16, 2010 at 12:36 PM
Anonymous said...

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

LSquared

March 16, 2010 at 4:40 PM

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