Domains of Rational Expressions

In this discussion, you are assigned two rational expressions to work on. Remember to factor all polynomials completely. Read the following instructions in order and view the example to complete this discussion. Please complete the following problems according to your assigned number. (Instructors will assign each student their number.)

If your assigned number is: Your first rational expression is Your second rational expression is
14 64k2 – 9
s2 + 2s – 15
s2 – 36
  • Explain in your own words what the meaning of domain is. Also, explain why a denominator cannot be zero.
  • Find the domain for each of your two rational expressions.
  • Write the domain of each rational expression in set notation (as demonstrated in the example).
  • Do both of your rational expressions have excluded values in their domains? If yes, explain why they are to be excluded from the domains. If no, explain why no exclusions are necessary.
  • Incorporate the following five math vocabulary words into your discussion. Use bold font to emphasize the words in your writing. Do not write definitions for the words; use them appropriately in sentences describing your math work.
    • Domain
    • Excluded value
    • Set
    • Factor
    • Real numbers

Your initial post should be at least 250 words in length. Support your claims with examples from required material(s) and/or other scholarly resources, and properly cite any references. Respond to at least two of your classmates’ posts by Day 7. Is their work similar to your own? Did they use the vocabulary words correctly? Do you understand their answers?




INSTRUCTOR GUIDANCE EXAMPLE: Week One Discussion Domains of Rational Expressions Students, you are perfectly welcome to format your math work just as I have done in these examples. However, the written parts of the assignment MUST be done about your own thoughts and in your own words. You are NOT to simply copy this wording into your posts! Here are my given rational expressions oh which to base my work. 25×2 – 4 67 5 – 9w 9w 2 – 4  The domain of a rational expression is the set of all numbers which are allowed to substitute for the variable in the expression. It is possible that some numbers will not be allowed depending on what the denominator has in it. In our Real Number System division by zero cannot be done. There is no number (or any other object) which can be the answer to division by zero so we must simply call the attempt “undefined.” A denominator cannot be zero because in a rational number or expression the denominator divides the numerator. In my first expression, the denominator is a constant term, meaning there is no variable present. Since it is impossible for 67 to equal zero, there are no excluded values for the domain. We can say the domain (D) is the set of all Real Numbers, written in set notation that would look like this: D = {x| x ∈ℜ} or even more simply as D = ℜ. For my second expression, I need to set the denominator equal to zero to find my excluded values for w. 9w2 – 4 = 0 I notice this is a difference of squares which I can factor. (3w – 2)(3w + 2) = 0 Set each factor equal to zero. 3w – 2 = 0 or 3w + 2 = 0 Add or subtract 2 from both sides. 3w = 2 or 3w = -2 Divide both sides by 3. w = 2/3 or w = -2/3  These are the excluded values for my second expression. The domain (D) for my second expression is the set of all Reals excluding ±2 /3. In set notation, this can be written as D = {w| w ∈ℜ, w ≠ ±2 /3} Now, both of my expressions do not have excluded values. In one expression, I have no excluded values because there is no variable in the denominator and a non-zero number will never just become zero. In the other expression, there are two excluded numbers because both, if inserted in place of the variable, would cause the denominator to become zero and thus the whole expression would become undefined.

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