Date: Oct 3, 2012 1:12 PM Author: Dave L. Renfro Subject: Re: How to simplify an expression ... Peter Duveen wrote (in part):

http://mathforum.org/kb/message.jspa?messageID=7900080

> I get something a bit different.

>

> I bring the r^2 up to the top, where it becomes r^-2.

> I then would tell the student to add the exponents where

> the base is the same. I think I interpreted the equation

> differently here, making t part of the exponent.

>

> So, combining the constants, we have [-1/2r^(8t-2)]^3

>

> We cube each factor. That means, in the case of the exponent,

> multiplying it by 3. One is left with:

>

> = -1/16r^(24t - 6)

>

> Did I do something amiss?

I didn't even think of that, and you might have the intended

interpretation, although given the way the spacing was in

[ (6 r^8 t) /(-3 r^2) ]^3

I still think (r^8)*t is more reasonable than r^(8t).

In any event, I also tell students about the sign change

method. I tell them that when you pass an exponentiated

expression through a division sign, the algebraic sign

of the exponent changes (negative to positive or positive

to negative, although almost always you do it in order to

change from negative to positive). Of course, you have to

point out that this is to only be done when the exponentiated

term is factored out of the entire side of the division sign,

and after you move it, it's (initially, at least) factored out

of the entire side of the other side of the division sign.

The method is quick, but it does tend to reduce the process

to a rote method rather than something students can see

themselves coming up with on their own. I tend to make more

of the long-and-drawn-out methods I posted about yesterday

when a student is first learning this stuff, and put more

emphasis on quickly getting to the desired result without

having to think very much when the student is more advanced

and the focus is on something else (e.g. calculus students

rewriting expressions after taking a derivative).

Incidentally, you can get students to practice working

intelligentally with exponents by using certain types of

no-calculator numerical evaluataion problems.

1. (0.00005)^(-8) = ??

Intended method: Rewrite as (0.5 x 10^(-4))^(-8), resulting in

(1/2)^(-8) x [10^(-4)]^(-8)

= 2^8 x 10^32

= 256 x 10^32 or 2.56 x 10^34

You can also do this via (5 x 10^(-5))^(-8), resulting in

10^40 / 5^8, at which point you can peal off eight 10's in

the numerator, writing each as 5*2, cancel the eight 5's

in the numerator with the eight 5's in the denominator,

leaving you with eight 2's and 10^32 in the numerator . . .

2. (0.0025)^3 / (0.05)^4 = ??

Intended method: Rewrite as [25 x 10^(-4)]^3 / [5 x 10^(-2)]^4,

which gives [(25)^3 x 10^(-12)] / [5^4 x 10^(-8)], or

(5^6 / 5^4) * (10^8 / 10^12) = . . .

3. Which of the following is closest to the number of digits

in the base-10 numeral expression for the value of 2^900?

90 180 270 450 900 1800

Intended method: 2^10 = 1024, which is approximately 10^3,

so 2^900 = (2^10)^90 is approximately (10^3)^90 = 10^270,

so 270 is the closest.

Dave L. Renfro