checking homework technique


My 12 year old son is learning algebra now, and while I'm a firm believer in the pencil and paper method for doing the homework, I also see this as a good opportunity for both of us to use our 33S to check the answers.

My question is, do you use your calculator's full capabilities to explicitly get an answer, or do you 'work with it' to try to simulate the pencil and paper method?

e.g. in multiplying rational numbers, we first multiply all the numbers and then count the number of negative signs to determine the sign of the product. So, (forgive the obviousness here) in:

-2 * 3 * -4

I teach him 2 * 3 is 6, * 4 is 24, and since there's an even number of negative signs, the answer is positive.

Now, checking with the 33s, would you key

2 [+/-] ENTER 3 ENTER 4 [+/-] * *

or would you just key

2 ENTER 3 ENTER 4 * *

and then just count the signs?

In the thread about the percent key someone made the point that 'the mathematically-inclined user is expected to be able to mentally shift the decimal point two places to the right.' Does that reasoning apply here?

Edited: 19 May 2007, 7:24 a.m. after one or more responses were posted


Personally I prefer to use the first method. Your example is very simple and thus most can do it even without the pencil and paper, but when you run into things that are more complicated, entering negative number as negative can prevent me from making mistakes. Anyway, I disagree that you're expected to shift the decimal point 2 places when using a calculator with the percent key.


My 11 year old son is in an accelerated math program...his teacher is really hitting the kids heavily with manipulation of equations, various tecniques for factoring polynomials, handling irrationals, radicals, etc. When we go over his homework, we work out the problems using the tecnique he was taught that day, but I usually check the solutions with my 50G (which of course does all the manipulation and simplification for you). He does use his 33S on rare occations, but mostly wades through it with pencil and paper.
When a calculator is called for in the classroom, the teacher passes out the classroom set of TI84's, but I understand that's seldom. I think when he starts doing trig, that will change.

Best regards, Hal

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