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Section 8.2 Algebraic Fractions

Subsection 1. Factor a polynomial

To reduce an algebraic fraction, we must factor its numerator and denominator.

Subsubsection Examples

Example 8.13.

Factor.

  1. \(\displaystyle 4x^2-4x\)
  2. \(\displaystyle 4x^2-1\)
  3. \(\displaystyle 4x^2-4x+2\)
Solution
  1. We factor out a common factor of \(4x\) to get \(4x(x-1)\text{.}\)

  2. This is a difference of two squares that factors as \((2x-1)(2x+1)\text{.}\)

  3. This is the square of a binomial, \((2x-1)^2\text{.}\)

Example 8.14.

Factor.

  1. \(\displaystyle 27a^2-3\)
  2. \(\displaystyle 27a^3-1\)
  3. \(\displaystyle 81a^3-a\)
Solution
  1. We first factor out 3 to find \(3(9a^2-1)\text{,}\) then factor the difference of two squares to get \(3(3a-1)(3a+1)\text{.}\)

  2. This is a differece of two cubes, which factors as \((3a-1)(9a^2+3a+1)\text{.}\)

  3. We first factor out \(a\) to get \(a(81a^2-1)\text{,}\) then factor the difference of two squares to get \(a(9a-1)(9a+1)\) .

Subsubsection Exercises

Factor completely \(2x^3+16y^3\)

Answer
\(2(x+2y)(x^2-2xy+4y^2)\)

Factor completely \(4x^2y-36y^3\)

Answer
\(4y(x-3y)(x+3y)\)

Factor completely \(2b^3-6b^2-36b\)

Answer
\(2b(b-6)(b+3)\)

Factor completely \(9b^4+9b^2\)

Answer
\(9b^2(b^2+1)\)

Subsection 2. Find the opposite of a binomial

To find the opposite or negative of a binomial we multiply by \(-1\text{.}\)

Subsubsection Examples

Example 8.19.

Which of these is the opposite of \(m^2-p~\text{?}\)

  1. \(\displaystyle m^2+p\)
  2. \(\displaystyle m-p^2\)
  3. \(\displaystyle p-m^2\)
Solution

The opposite of \(m^2-p\) is \(-(m^2-p) = -m^2+p\text{,}\) or \(p-m^2\text{.}\)

Example 8.20.

Which of these pairs of binomials are opposites?

  1. \(3c-5\) and \(5+3c\)
  2. \(5-3c\) and \(3-5c\)
  3. \(5c-3\) and \(3-5c\)
Solution

The opposite of \(5c-3\) is \(-(5c-3)=-5c+3\text{,}\) or \(3-5c\text{,}\) so (c) is correct.

Subsubsection Exercises

Find the opposite of the binomial \(2x+1\)

Answer
\(-2x-1\)

Find the opposite of the binomial \(b^2-b\)

Answer
\(b-b^2\)

Find the opposite of the binomial \(-4n+8\)

Answer
\(4n-8\)

Find the opposite of the binomial \(-3z^2-2\)

Answer
\(3z^2+2\)

Subsection 3. Use horizontal and vertical lines

The asymptotes of rational functions are horizontal and vertical lines.

Subsubsection Examples

Example 8.25.

Give the equation and slope of the line.

vertical line
Solution

This line is a vertical line. All the points on the line have \(x\)-coordinate \(-15\text{,}\) so the equation of the line is \(x=-15\text{.}\) Because \(\Delta x=0\) between any two points on the line, its slope is undefined.

Example 8.26.

Give the equation and slope of the line.

horizontal line
Solution

This line is a horizontal line. All the points on the line have \(y\)-coordinate \(04\text{,}\) so the equation of the line is \(y=40\text{.}\) Because \(\Delta y=0\) between any two points on the line, its slope is \(0\text{.}\)

Subsubsection Exercises

Give the equation and slope of the line.

horizontal line
Answer
\(y=-16;~ m=0\)

Give the equation and slope of the line.

vertical line
Answer
\(x=30;~ m~\)is undefined

Give the equation and slope of the line.

horizontal line
Answer
\(y=45; m=0\)

Give the equation and slope of the line.

vertical line
Answer
\(x=-24;~ m~\)is undefined