Last Minute Math 2C Questions!!!!

IlDesi
Fri, 06 Jun 2008 20:49:22 +0000

Descartes Rule of Signs is just about useless but I can explain it anyway.
You have a function, say f(x) = x^3 + 5x^2 -3x +2
Now you see how many sign changes (positive to negative) there are (so 5x^2 to -3x is a sign change as is -3x to 2). You have two sign changes, so there are either 2 positive roots or 0 positive roots (subtract by two)

Now you evaluate f(-x) which ends up being -x^3 + 5x^2 + 3x + 2
Again see how many sign changes there are (only one this time). This means you have 1 negative root (can’t subtract by two because there’s only one)

Hope that helped. I’m bad at explaining without a piece of paper =p