Saturday, November 13, 2010

Elijah's ScribePost for November 13, 2010

For Math, my question was number 17, Chapter 2.3

17. To calculate -3/4 + (-2/3), Amy decided to convert the fractions to decimals and add the decimals on a scientific calculator.

a) Explain why she had difficulty in determining the exact answer by this method.
For Amy's method, she wanted to turn - 3/4 and -2/3 into decimals and just add them up.

If you convert - 3/4 into a decimal it would be - 0.75.

If you convert - 2/3 into a decimal it would be -0.6 repeating.

If you add those two up you can't get the exact answer because there is a repeating decimal in the question.

b) How should she calculate to get an exact answer?

Amy should just stick with adding it the fractions method.

- 3/4 + (-2/3)=

Well first you have to find the common denominator for the fraction. Their common denominator is 12. And how do you turn the denominator ( 4 ) and ( 3) to 12 ? You multiply 4 by 3 and you multiply the 3 by 4. Now the fractions denominator is 12. But you also have to do the same to both fractions numerator. So you multiply the 3 by 3 which is 9 and multiply the 2 by 4 which is 8. The fractions are now -9/12 and -8/12. Then you just add the 9 and 8 together and which becomes the numerator and the denominator stays the same.

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