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Topic: How to simplify an expression ...
Replies: 4   Last Post: Oct 3, 2012 10:29 PM

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Dave L. Renfro

Posts: 4,792
Registered: 12/3/04
Re: How to simplify an expression ...
Posted: Oct 2, 2012 4:43 PM
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> I've noticed through the years that many students had
> considerable difficulty simplifying the expression below
> on a test in Intermediate Algebra:
> [ (6 r^8 t) /(-3 r^2) ]^3
> I'm interested in seeing how you think students should
> approach this problem, and detailed steps they should
> take, along with precise assumptions/formulas they
> should know and be able to apply correctly at each step.
> Thank you in advance.

I would tell them that if a fraction anywhere in sight
(that isn't being added to or subtracted from another fraction)
can clearly be reduced, then they should first reduce the
fraction. This assumes, of course, that students can
recognize when a fraction can be easily reduced and that
they can easily carry out the reduction of the fraction.
If this is a problem for students, then I would focus
on working on just that aspect, and not getting into
something like the above.

Doing this leads us to [-2*r^6*t]^3, which is just
[-2 * r^6 * t] * [-2 * r^6 * t] * [-2 * r^6 * t]
= [-2 * -2 * -2] * [r^6 * r^6 * r^6] * [t*t*t]
= (-2)^3 * (r^6)^3 * t^3
= -8 * r^18 * t^3.

I've found that it helps if you often fill in the step of
replacing an exponent with repeated multiplication,
rather than simply using a rule that some students may still
be unsure of. In the calculation above you can then go back
and show how our work essentially verifies the identity
(ABC)^3 = A^3 * B^3 * C^3. Certainly do this when students
are still learning and practicing properties of exponents,
although I've even found it helpful in calculus courses
from time to time. Also, I might (on a blackboard) use
a different color of chalk and, off to the side with an
arrow drawn to the appropriate place, write-and-box something
like this: r^6 * r^6 * r^6 = (rrrrrr) * (rrrrrr) * (rrrrrr),
which is a total of 18 r's being multiplied.

As for reducing fractions, here's an approach I often use
when explaining the process to students.

Break up the fraction into separate fractions being
multiplied (remind students how one multiplies fractions)
so that like things are with like things. I'm reminded of
the cafeteria scene in the 2004 movie "Mean Girls" where one
person says: "Where you sit in the cafeteria is crucial
'cause you got everybody there. You've got your Freshmen,
ROTC guys, Preps, JV Jocks, Asian Nerds, Cool Asians, Varsity
Jocks, Unfriendly Black Hotties, Girls Who Eat Their Feelings,
Girls Who Don't Eat Anything, Desperate Wannabes, Burnouts,
Sexually Active Band Geeks, The Greatest People You Will Ever
Meet, and The Worst. Beware of the Plastics." However, it's
probably not a good idea to bring up this in an actual math
classrooom (or you'll find that, among the many later retellings
of this by students to their parents and friends, enough of
the original context will be lost in a few cases that you'll
be accused of advocating racism), but one-on-one tutoring
might be O-K.

Doing this to (6 r^8 t) / (-3 r^2) leads to

(6 / -3) * (r^8 / r^2) * (t / 1)

Of course, the fractions would be written in vertical form.

Now reduce each separate fraction:

- -(2 / 1) * (r^6 / 1) * (t / 1)

= (-2 * r^6 * t) / 1*1*1

= -8 * r^6 * t

Finally, I might (on a blackboard) use a different color of
chalk and, off to the side with an arrow drawn to the appropriate
place, write-and-box something like this:

r^8 / r^2

= (rrrrrrrr) / (rr)

= (r/r) * (r/r) * (r/r) * (r/r) * (r/r) * (r/r) * (r/1) * (r/1)

= 1*1*1*1*1*1*r*r

= r^2

One of the things I'd hope to get across with this extra
work is that you can often do these calculations even if
you've forgotten some exponent rules. Just write down what
everything is without using exponents. Also, it shows that
these exponent rules aren't arbitrary "math grammar" rules,
but rather these rules simply represent shortcuts one can
notice, at least in the case for positive integer exponents.

Dave L. Renfro

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