Synthetic Division & Finding Zeroes (page 3 of 4) Sections: Introduction, Worked examples, Finding zeroes, Factoring polynomials Once you know how to do synthetic division, you can use the technique as a shortcut to finding factors and zeroes of polynomials. Here are some examples:
Set up the synthetic division, and check to see if the remainder is zero. If the remainder is zero, then x = 1 is a zero of x^{3} – 1. To do the initial setup, note that I needed to leave "gaps" for the powers of x that are not included in the polynomial. That is, I followed the practice used with long division, and wrote the polynomial as x^{3} + 0x^{2} + 0x – 1 for the purposes of doing the division. If you forget to leave "gaps", your division will not work properly!
Since the remainder is zero, then x = 1 is a zero of x^{3} – 1. Since x
= 1 is a zero of x^{3}
– 1, then x
– 1 is a factor, so
the polynomial x^{3}
– 1 factors as
Comparing the results of the Rational Roots Test to a quick graph, I decide to test x = 2 as a possible zero. Set up the divison: ...and here is the result: Since the remainder is zero, then x = 2 is indeed a zero of the original polynomial. To continue on and find the rest of the zeroes, should I start over again with x^{4} + x^{3} –11x^{2} – 5x + 30? Well, think about when you factor something like 72. After you divide a 2 out and get a 36, do you go back to the 72 to try the next factor, or do you see what will go into the 36? Of course, you try factors into the 36. Follow the same procedure here. I won't return to the original polynomial, but will instead see what divides into my result. (Recall that syntheticdividing out x = 2 is the same as longdividing out x – 2, so the result has a degree that is one lower than what I started with. That is, to continue, I will be dealing not with the original fourthdegree polynomial x^{4} + x^{3} –11x^{2} – 5x + 30, but with the thirddegree result from the synthetic division: x^{3} + 3x^{2} – 5x – 15.) Continuing, and again comparing the Rational Roots Test with a quick graph, I will try x = –3. Set up the division: ...and here is the result: Since the remainder is zero, then x = –3 is a zero of the original polynomial. At this point, the final result is a quadratic, (x^{2} – 5), and I can apply the Quadratic Formula or other methods to get the remaining zeroes: Then all the zeroes are: The above example shows how synthetic division is mostcommonly used: You are given some polynomial, and told to find all of its zeroes. You create a list of possibilities, using the Rational Roots Test; you plug various of these possible zeroes into the synthetic division until one of them "works" (divides out evenly, with a zero remainder); you then try additional zeroes on the resulting (and lowerdegree) polynomial until something else "works"; and you keep going like this until you get down to a quadratic, at which point you use the Quadratic Formula or other methods to get the last two of the original polynomial's zeroes. << Previous Top  1  2  3  4  Return to Index Next >>



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