SOLUTION: Indentify the number of positive, negative, and imaginary roots, all rational roots , and the total amount of roots. x^3+x-6=0

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Question 1134148: Indentify the number of positive, negative, and imaginary roots, all rational roots , and the total amount of roots.
x^3+x-6=0

Found 2 solutions by MathLover1, ikleyn:
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!
x%5E3%2Bx-6=0+
=> the highest degree of its monomial (individual term) is 3, so this function will have one to three roots

use the Newton-Raphson method:
x%5Bn%2B1%5D=x%5Bn%5D-f%28x%5Bn%5D%29/f'%28x%5Bn%5D%29

f'%28x%5Bn%5D%29=3x%5E2%2B1
let x%5B0%5D=5
compute x%5Bn%2B1%5D until \Delta \:x%5Bn%2B1%5D+%3C+0.000001
f%28x%5B0%5D%29=5%5E3%2B5-6=124
f'%28x%5B0%5D%29=3%2A5%5E2%2B1=76
x%5B1%5D=5-124%2F76
x%5B1%5D=5-124%2F76
x%5B1%5D=3.36842
Δx%5B1%5D=5-3.36842
Δx%5B1%5D=highlight%281.6344%29=> one real root

Apply long division:
%28x%5E3%2Bx-6%29%2F%28x-1.6344%29+=x%5E2%2B1.6344x%2B3.67115

x+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+

x+=+%28-1.6344+%2B-+sqrt%28+1.6344%5E2-4%2A1%2A3.67115+%29%29%2F%282%2A1%29+
x+=+%28-1.6344+%2B-+sqrt%28+-12.01333664+%29%29%2F2+
x+=+%28-1.6344+%2B-+3.466%2Ai%29%2F2+

other roots:
x+=+%28-1.6344+%2B+3.466%2Ai%29%2F2+ =>highlight%28x%5B2%5D+=-0.8172%2B1.733%2Ai%29
x+=+%28-1.6344+-+3.466%2Ai%29%2F2+ =>highlight%28x%5B3%5D+=-0.8172+-1.733%2Ai%29


Answer by ikleyn(52776) About Me  (Show Source):
You can put this solution on YOUR website!
.
The polynomial on the left side is of degree 3 - so the total amount of the roots is 3.


It includes real and complex number roots.


The derivative of the left side polynomial is  3x^2 + 1; from the first glance,

it is clear that this derivative function is always positive - so the original 

polynomial function on the left side is monotonically increasing.


It means, that there is ONLY ONE real root.  Hence, other two roots are complex numbers.


At x= 0, the left side function has the value of -6, and it rises monotonically to +infinity as  x --> oo   - 

    hence, the real root is a positive real number. 


Then, according to the Rational Root theorem, rational roots can be only among these numbers (divisors of 6): 1, 2, 3 or 6.


But easy mental check shows that the number 2 is not the root, and, moreover, at x= 2 the left side function is just positive, 
so the only real root is IRRATIONAL number between 1 and 2.


So, the required analysis is COMPLETED, and its results are as follows:


    - total number of roots is 3;

    - there is only one real root;

    - there are two complex roots;

    - the real root is positive;

    - the number of positive real roots is 1;

    - the number of negative real roots is zero;

    - the number of rational real roots is zero.


All the questions are answered;  so the solution is completed.