# subtracting polynomials

• Mar 25th 2008, 10:47 AM
subtracting polynomials
7x+1 - x^2+3x-8
i got x^2-4x+9 but my teacher said it was wronge. how do I correctly solve this problem?
• Mar 25th 2008, 11:06 AM
Moo
Hello,

Where did the - go for x² ?
• Mar 25th 2008, 11:28 AM
I am not sure why would the x^2 be negative i am so lost
• Mar 25th 2008, 11:41 AM
mathceleb
Quote:

I am not sure why would the x^2 be negative i am so lost

Are you subtracting the entire last 3 terms? If not, the original expression you entered was:

\$\displaystyle 7x + 1 - x^2 + 3x - 8\$

Grouping like terms and evaluating, we get:

\$\displaystyle (7 + 3)x + (1 - 8) - x^2\$
\$\displaystyle 10x -7 - x^2\$

Rearranging this into proper form, we get:
\$\displaystyle -x^2 + 10x -7\$

Does that make sense?
• Mar 25th 2008, 11:45 AM
there where only 2 polnomials 7x+1 minus x^2+3x-8
• Mar 25th 2008, 11:47 AM
mathceleb
Quote:

there where only 2 polnomials 7x+1 minus x^2+3x-8

ok, then you should have written it like this:

\$\displaystyle 7x + 1 - (x^2 + 3x - 8)\$

Now, distribute the negative sign for the second polynomial:

\$\displaystyle 7x + 1 - x^2 - 3x + 8\$

Now group like terms:
\$\displaystyle (7 - 3)x + (1 + 8) - x^2\$
\$\displaystyle -x^2 + 4x + 9\$
• Mar 25th 2008, 12:05 PM