SOLUTION: Please help me solve this equation. {{{9x^2-17x^4+21x^5}}}

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Question 116758This question is from textbook Intermediate Algebra
: Please help me solve this equation. 9x%5E2-17x%5E4%2B21x%5E5 This question is from textbook Intermediate Algebra

Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!
This is not an equation. This is a polynomial expression. As it stands, there is no solution -- you can't 'solve' a polynomial expression.

So, let's presume that you actually meant to say 'Please solve: 9x%5E2-17x%5E4%2B21x%5E5=0.

The first thing you can do is factor x%5E2 out of the left side leaving you with:

x%5E2%2821x%5E3-17x%5E2%2B9%29=0

That gives us two of the five roots directly, namely x=0 and x=0

From here on it gets mighty ugly. Looking at a graph of the cubic factor, we can see that we have one real root, slightly smaller than -%281%2F2%29 and a conjugate pair of complex roots.

graph%28600%2C600%2C-4%2C4%2C-20%2C20%2C21x%5E3-17x%5E2%2B9%29

My recommendation, to the extent that your instructor will be satisfied with a reasonable approximation, is to go to a website that has a general cubic equation solver. When you use such a thing, just be sure to remember that the c coefficient in ax%5E3%2Bbx%5E2%2Bcx%2Bd=0 has a zero value in this problem.

There is another possibility that occurred to me -- you may have a typo in your polynomial. If you actually meant 9x%5E3-17x%5E4%2B21x%5E5, then 9x%5E3-17x%5E4%2B21x%5E5=0 becomes:

x%5E3%2821x%5E2-17x%2B9%29 and we are left with a much easier to handle quadratic.

Solved by pluggable solver: SOLVE quadratic equation with variable
Quadratic equation ax%5E2%2Bbx%2Bc=0 (in our case 21x%5E2%2B-17x%2B9+=+0) has the following solutons:

x%5B12%5D+=+%28b%2B-sqrt%28+b%5E2-4ac+%29%29%2F2%5Ca

For these solutions to exist, the discriminant b%5E2-4ac should not be a negative number.

First, we need to compute the discriminant b%5E2-4ac: b%5E2-4ac=%28-17%29%5E2-4%2A21%2A9=-467.

The discriminant -467 is less than zero. That means that there are no solutions among real numbers.

If you are a student of advanced school algebra and are aware about imaginary numbers, read on.


In the field of imaginary numbers, the square root of -467 is + or - sqrt%28+467%29+=+21.6101827849743.

The solution is

Here's your graph:
graph%28+500%2C+500%2C+-10%2C+10%2C+-20%2C+20%2C+21%2Ax%5E2%2B-17%2Ax%2B9+%29



Not nearly as messy, even though the roots are complex.


Hope this helps.