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zerlinav
Group Title
What are the possible number of positive, negative, and complex zeros of f(x) = 3x4  5x3  x2  8x + 4
 2 years ago
 2 years ago
zerlinav Group Title
What are the possible number of positive, negative, and complex zeros of f(x) = 3x4  5x3  x2  8x + 4
 2 years ago
 2 years ago

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richyw Group TitleBest ResponseYou've already chosen the best response.1
Ok well first of all it's a quadratic function, so the total will be 4. Use Descartes’ Rule of Signs. Your function is \[p(x)=3x^45x^3x^28x+4\] So count the number of sign changes. A "sign change" is when the sign the coefficient of the \(a^n\) is different from the sign of the coefficient of \(a^{n1}\). So here you have one sign changes. You do the same thing for \(p(x)\) and also find one sign change (don't include the sign change for the 4! only the ones that multiply by x matter. Since there must be 4 total you know that it is one positive, one negative and two complex (the complex ones have to be an even number!)
 2 years ago

richyw Group TitleBest ResponseYou've already chosen the best response.1
sorry not quadratic. haha. 4th degree polynomial
 2 years ago

richyw Group TitleBest ResponseYou've already chosen the best response.1
or "quartic (I just had to google that, lol)
 2 years ago

cruffo Group TitleBest ResponseYou've already chosen the best response.1
I thought there were two sign changes. \[ p(x)=3x^45x^3x^28x+4 \rightarrow \color{red}{+}\color{red}{} \color{red} \color{red}+\]
 2 years ago

cruffo Group TitleBest ResponseYou've already chosen the best response.1
# of possible positive zeros is the number of sigh changes of the polynomial, or less than that number by an even integer. So the number of possible positive zeros is 2 or 0
 2 years ago

richyw Group TitleBest ResponseYou've already chosen the best response.1
there is only one sign change though. That last one doesn't matter.
 2 years ago

cruffo Group TitleBest ResponseYou've already chosen the best response.1
??? http://www.purplemath.com/modules/drofsign.htm
 2 years ago

richyw Group TitleBest ResponseYou've already chosen the best response.1
weird, I might be wrong. But I don't think so.
 2 years ago

cruffo Group TitleBest ResponseYou've already chosen the best response.1
Take a look at the graph: http://www.wolframalpha.com/input/?i=p%28x%29%3D3x^45x^3x^28x%2B4+from+3+to+3
 2 years ago

richyw Group TitleBest ResponseYou've already chosen the best response.1
oh yeah you are totally correct!
 2 years ago

richyw Group TitleBest ResponseYou've already chosen the best response.1
so in this case using the rule of signs all that you can know is one of four possible combinations then right? Also I have been doing this wrong for like 5 years haha. good thing we have calculators...
 2 years ago

cruffo Group TitleBest ResponseYou've already chosen the best response.1
: ) Sounds right: degree 4 means you will find 4 solutions to p(x) = 0 For this polynomial, just looking at the signs, we can tell that there will be 2 or 0 positive roots, 2 or 0 negative roots. For the complex roots  they always come in pairs. so... case 1: 2 + real zeros, 0  real zeros, and 2 complex zeros case 2: 0 + real zeros, 2  real zeros, and 2 complex zeros case 3: 2 + real zeros, 2  real zeros, and 0 complex zeros case 4: 0 + real zeros, 0  real zeros, and 4 complex zeros
 2 years ago
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