Precalculus : Real Numbers

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #1 : Real Numbers

Which of the following is not true about a field. (Note: the real numbers \displaystyle \small \mathbb{R} is a field)

Possible Answers:

A field can be defined in many ways.

For every element \displaystyle \small a in the field, there is another element \displaystyle \small b such that their product \displaystyle \small \small ab  is equal to \displaystyle \small 1, where \displaystyle \small 1 is the multiplicative identity, \displaystyle \small 1 in the case of real numbers.

We have \displaystyle \small ab=ba for any \displaystyle \small a and \displaystyle \small b in the field.

There is an element \displaystyle \small e in the field such that \displaystyle \small e+a=a+e=a for any element \displaystyle \small a in the field.

For every element \displaystyle \small a in the field, there is another element \displaystyle \small b such that their sum \displaystyle \small a+b is equal to \displaystyle \small 0, where \displaystyle \small 0 is the additive identity. 

Correct answer:

For every element \displaystyle \small a in the field, there is another element \displaystyle \small b such that their product \displaystyle \small \small ab  is equal to \displaystyle \small 1, where \displaystyle \small 1 is the multiplicative identity, \displaystyle \small 1 in the case of real numbers.

Explanation:

It is not the case that for any element \displaystyle \small a in a field, there is another one \displaystyle \small b such that their product is \displaystyle \small 1. Take \displaystyle \small 0 in the real numbers. Multiply \displaystyle \small 0 by any number and you get \displaystyle \small 0, so you will never get \displaystyle \small 1. This is true for any field that has more than 1 element.

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