SOLUTION: Use Descartes rule of signs to find the possible positive, negative, and imaginary zeroes for: x^5 − 3x^4 + x^3 − 4x^2 + 5x − 1

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Question 1154820: Use Descartes rule of signs to find the possible positive, negative, and imaginary zeroes for:
x^5 − 3x^4 + x^3 − 4x^2 + 5x − 1

Found 2 solutions by greenestamps, MathLover1:
Answer by greenestamps(13200) About Me  (Show Source):
You can put this solution on YOUR website!


With the polynomial in standard form (in decreasing powers of the variable), there are 5 sign changes. So the number of positive real roots is 5, or 3, or 1.

Replacing "x" with "-x" in the polynomial results in a polynomial with 0 sign changes, so there are no negative real roots.

A graph shows that in fact there are 3 positive real roots, which means there is 1 pair of imaginary roots.

graph%28400%2C400%2C-2%2C4%2C-5%2C5%2Cx%5E5-3x%5E4%2Bx%5E3-4x%5E2%2B5x-1%29


Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!
x%5E5+-3x%5E4+%2B+x%5E3+-4x%5E2+%2B+5x+-+1
Here are the coefficients of our variable x:
1..... -3..... 1..... +-4..... +5+..... -1
As can be seen, there are 5 changes.
This means that there are 5 or+3 or 1 positive real roots.
To find the number of negative real roots, substitute x with -x in the given polynomial:

x%5E5+-3x%5E4+%2B+x%5E3+-4x%5E2+%2B+5x+-+1 becomes -x%5E5-3x%5E4-x%5E3-4x%5E2-5x-1
The coefficients are -1,-3,-1,-4,-5,-1.

As can be seen, there are+0 changes. This means that there are 0 negative+real roots.

+graph%28+600%2C+600%2C+-5%2C+5%2C+-5%2C+5%2C+x%5E5+-3x%5E4+%2B+x%5E3+-4x%5E2+%2B+5x+-+1%29+

graph shows 3 positive real roots, means there will be one pair of imaginary roots too