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Grade 12

Transformations of rational functions

Lesson

Just as a polynomial function $y=P\left(x\right)$y=P(x) can be transformed, so also can rational functions, given by  $y=\frac{P\left(x\right)}{Q\left(x\right)}$y=P(x)Q(x), be transformed. Remember that transformation of a function just means to change it, we can transform a function using a translation (shift vertically or horizontally), dilation (expansion horizontally or vertically), or reflection (across an axis or other given line). 

For example:

  • The rational function $y=\frac{x-2}{x+5}$y=x2x+5, expressible as $y=\frac{\left(x-2\right)}{\left(x-2\right)+7}$y=(x2)(x2)+7, can be thought of as the simpler function $y=\frac{x}{x+7}$y=xx+7 translated $2$2 units to the right.
  • The rational function $y=\frac{2x^3}{x^2+5}+3$y=2x3x2+5+3 is similarly the function $y=\frac{2x^3}{x^2+5}$y=2x3x2+5 translated upwards by $3$3 units.
  • The rational function $y=\frac{-12x}{x+1}$y=12xx+1 is the function $y=\frac{x}{x+1}$y=xx+1 reflected in the $x$x axis and dilated by a factor of $12$12.

 

An Example

Find the equation of the transformed function created when the rational function $y=\frac{x^2}{x-1}$y=x2x1 is first translated $3$3 units to the right and $5$5 units upward, and then dilated by a factor of $2$2.

Then the rational function $y=\frac{x^2}{x-1}$y=x2x1 after translation becomes $y=\frac{\left(x-3\right)^2}{\left(x-3\right)+1}+5$y=(x3)2(x3)+1+5, and then after dilation becomes $y=\frac{2\left(x-3\right)^2}{\left(x-3\right)+1}+5$y=2(x3)2(x3)+1+5

Note that the dilation does not influence the translation. 

The transformed function can then be simplified to $y=\frac{2\left(x-3\right)^2}{x-2}+5$y=2(x3)2x2+5. The graph shows the two stages of transformation.

As an example, the point with coordinates $\left(-2,4\right)$(2,4) on the original green curve translates to the point $\left(1,1\right)$(1,1) on the blue curve. Then, after the dilation, the point changes again to the point $\left(1,-3\right)$(1,3) on the red transformed curve.

 

  

 

 

 

 

Outcomes

12F.C.2.2

Determine, through investigation with and without technology, key features of the graphs of rational functions that have linear expressions in the numerator and denominator and make connections between the algebraic and graphical representations of these rational functions

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