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Rational Functions In this chapter, you’ll learn what a rational function is, and you’ll learn how to sketch the graph of a rational function. Rational functions A rational function is a fraction of polynomials. That is, if p(x) and q(x) are polynomials, then p(x) q(x) is a rational function. The numerator is p(x) and the denominator is q(x). Examples. 3(x 5) • (x 1) • x1 3 • 2x1 = 2x3 The last example is both a polynomial and a rational function. In a similar way, any polynomial is a rational function. In this class, from this point on, most of the rational functions that we’ll see will have both their numerators and their denominators completely factored. We will also only see examples where the numerator and the denominator have no common factors. (If they did have a common factor, we could just cancel them.) * * * * * * * * * * * * * Implied domains The implied domain of a rational function is the set of all real numbers except for the roots of the denominator. That’s because it doesn’t make sense to divide by 0. 197 Example. Example. The Theimplied implieddomain domainofof 2 7(x 7(x 2)(x 2)(x2++1) 1) 8(x 8(x 4)(x 4)(x 6) 6) isisthe theset setRR {4, {4,6}. 6}. Vertical Vertical asymptotes asymptotes To Tograph graphaarational rationalfunction, function,begin beginby bymarking markingevery everynumber numberon onthe thex-axis x-axis that is a root of the denominator. (The denominator might not have that is a root of the denominator. (The denominator might not have any any roots.) roots.) Draw Drawaavertical verticaldashed dashedline linethrough through these these points. points. These These vertical vertical lines lines are are called calledvertical verticalasymptotes. asymptotes. The Thegraph graphofofthe therational rationalfunction functionwill will“climb “climbup” up” or “slide down” the sides of a vertical asymptote. or “slide down” the sides of a vertical asymptote. 1 Examples. Examples. For Forthe therational rationalfunction functionxx1, ,00isisthe theonly onlyroot rootofofthe thedenominator, denominator, 1 so sothe they-axis y-axisisisthe thevertical verticalasymptote. asymptote. Notice Noticethat thatthe thegraph graphofof xx1 climbs climbsup up the right side of the y-axis and slides down the left side of the y-axis. the right side of the y-axis and slides down the left side of the y-axis. The Therational rationalfunction function 2 7(x 7(x 2)(x 2)(x2++1) 1) 8(x 8(x 4)(x 4)(x 6) 6) has hasvertical verticalasymptotes asymptotesat atxx==44and andat atxx==6.6. ** ** ** ** ** ** ** 152 198 ** ** ** ** ** ** x-intercepts The x-intercepts of a rational function p(x) q(x) (if there are any) are the numbers ↵ 2 R where p(↵) =0 q(↵) If ↵ is such a number, then we can multiply by q(↵) to find that p(↵) = 0 · q(↵) = 0 In other words, ↵ is a root of p(x). Thus, the roots of the numerator are exactly the x-intercepts. Example. 2 is the only x-intercept of the rational function 7(x 8(x * * * * * 2)(x2 + 1) 4)(x 6) * * * * * * * * In between x-intercepts and vertical asymptotes When graphing a rational polynomial, first mark the vertical asymptotes and the x-intercepts. Then choose a number c 2 R between any consecutive pairs of these marked points on the x-axis and see if the rational function is positive or negative when x = c. If it’s positive, draw a dot above the x-axis whose first coordinate is c. If it’s negative, draw a dot below the x-axis whose first coordinate is c. Example. Let’s look at the function 7(x 2)(x2 + 1) r(x) = 8(x 4)(x 6) again. The x-intercept of its graph is at x = 2 and it has vertical asymptotes at x = 4 and x = 6. We need to decide whether r(x) is positive or negative between 2 and 4 on the x-axis, and between 4 and 6 on the x-axis. Let’s start by choosing a number between 2 and 4, say 3. Then 7(3 r(3) = 8(3 199 2)(32 + 1) 4)(3 6) Notice that 7, (3 4), and (3 6) are negative, while 8, (3 2), and (32 + 1) are positive. If you are multiplying and dividing a collection of numbers that aren’t equal to 0, just count how many negative numbers there are. If there is an even number of negatives, the result will be positive. If there is an odd number of negatives, the result will be negative. In the previous paragraph, there are three negative numbers — 7, (3 4), and (3 6) — so r(3) < 0. The number 5 is a number that is in between 4 and 6, and * * * 7(5 r(5) = 8(5 2)(32 + 1) >0 4)(5 6) * * * * * * * * * * Far right and far left Let axn be the leading term of p(x) and let bxm be the leading term of q(x). Recall that far to the right and left, p(x) looks like its leading term, axn . And far to the right and left, q(x) looks like its leading term, bxm . It follows that the far right and left portion of the graph of, p(x) q(x) looks like axn bxm and this is a function that we know how to graph. Example. The leading term of 7(x 2)(x2 + 1) is 7x3 , and the leading term of 8(x 4)(x 6) is 8x2 . Therefore, the graph of 7(x r(x) = 8(x looks like the graph of 2)(x2 + 1) 4)(x 6) 7x3 7 = x 8x2 8 on the far left and far right part of its graph. 200 ** ** ** ** ** ** ** ** ** ** ** ** ** Putting the the graph graph together together Putting To graph graph aa rational rational function function To p(x) p(x) q(x) q(x) mark its its vertical vertical asymptotes asymptotes (if (if any). any). Mark Mark its its x-intercepts x-intercepts (if (if any). any). DeterDetermark mine whether whether the the function function isis positive positive or or negative negative in in between between x-intercepts x-intercepts and and mine vertical asymptotes. asymptotes. vertical Replace p(x) p(x) with with its its leading leading term, term, replace replace q(x) q(x) with with its its leading leading term, term, and and Replace then graph graph the the resulting resulting fraction fraction of of leading leading terms terms to to the the right right and and left left of of then everything you’ve you’ve drawn drawn so so far far in in your your graph. graph. everything Now draw draw aa reasonable reasonablelooking lookinggraph graphthat thatfits fitswith witheverything everythingyou’ve you’vedrawn drawn Now so far, far, remembering remembering that that the the graph graph has has to to climb climb up up or or slide slide down down the the sides sides so of vertical vertical asymptotes, asymptotes, and and that that the the graph graph can can only only touch touch the the x-axis x-axis at at the the of x-intercepts that that you you already already marked. marked. x-intercepts Example. Let’s Let’s graph graph Example. 7(x 2)(x 2)(x22 + +1) 1) 7(x r(x) = = r(x) 8(x 4)(x 4)(x 6) 6) 8(x First we we mark mark its its its its vertical vertical asymptotes, asymptotes, which which are are at at xx = = 44 and and xx = = 6, 6, First and its its x-intercept, x-intercept, which which isis at at xx = = 2. 2. and a I I I I I I I a I 61 I I I I I I I I I a I 155 201 I I I I Then we plot points that represent what we had checked earlier for what Then we we plot plot points points that that represent represent what what we we had had checked checked earlier earlier for for what what Then happens in between consecutive pairs of x-intercepts and vertical asymptotes: happens in in between between consecutive consecutive pairs pairs of of x-intercepts x-intercepts and and vertical vertical asymptotes: asymptotes: happens that r(3) < 0 and r(5) > 0. that r(3) r(3) < < 00 and and r(5) r(5) > > 0. 0. that I I I I 2. S I I I I I I I I I I I I Lbi I I I I I I I I I I To the left and right of what we’ve graphed so far, we draw the graph of To the the left left and and right right of of what what we’ve we’ve graphed graphed so so far, far, we we draw draw the the graph graph of of To a I I I I I I I I I I 6’ 3 0 202 156 156 I I I I 7 877x. x. 88 x. Now Now we we connect connect what what we’ve we’ve drawn drawn so so far, far, making making sure sure our our graph graph climbs climbs up up or slides down the vertical asymptotes, and that it only touches the x-axis or slides down the vertical asymptotes, and that it only touches the x-axis at at the previously labelled x-intercept. the previously labelled x-intercept. II a rfr) I’ * * * * * * * * * * * * * * 203 157 * * * * * * * * * * * * Exercises For #1-3, use that 4x2 4 = 4(x 1)(x + 1), x3 3x2 + 4 = (x + 1)(x 2)2 , and 2x 4 = 2(x 2) to match each of the three numbered rational functions on the left with its simplified lettered form on the right. 2 1.) x34x3x24+4 A.) 12 (x + 1)(x 2.) x3 3x2 +4 2x 4 B.) (x 2)2 3.) 2x 4 4x2 4 C.) 2(x 1)(x+1) 2) 4(x 1) (x 2) Graph the rational functions given in #4-10. (Their numerators and denominators have been completely factored.) Then match each graph with one of the lettered graphs drawn on the next two pages. 4.) 3(x2 +1) (x2 +5) 8.) (x2 +3)(x2 +4)(x2 +8) 5.) 4(x+1)2 2(x+2)(x 2) 9.) 5(x 2)2 (x 6) 6.) (x 4)(x 6) 3(x2 +7) (x+1)(x2 +1)(x2 +8) (x 7) 7.) 7(x + 2)3 (x 10.) 2(x+10)2 (x+30) 3(x 5) 3)2 11.) Completely factor the numerator and the denominator of the rational function below, and then graph it. Match the graph with one of those on the next two pages. 3x3 6x2 + x 2 x2 + 3x + 2 204 2-06 A.) B.) 2-06 -s C.) D.) 4 -10 Nñ 205 Nñ E.) F.) 4 Nñ -10 G.) H.) 2-06 Nñ -s 4 206