Multiplying biominals

[Guest]

(x-6)(9x+4)(9x-4)

Using the FOIL method on the first two I came up with

9x^2+4x-54x+24(9x-4)

9x(9x^2+4x-54x+24)

81x^3+36x^2-486x^2-216x
81x^3-450x^2-216x
-4(9x^2+4x-54x+24)
-36x^2-16x+216x+96
-36x^2+200x+96

Then I combined like terms
81x^3-486x^2-16x+96

Correct?

 

[stapel]

No.

First, check your work on multiplying x - 6 by 9x + 4. I'm pretty sure (-6)(+4) doesn't equal +24. And you need to simplify by combining "like" terms.

You need to be more careful with your parentheses. Whatever might be the result of the above multiplication, 9x - 4 is multiplied on the entire result, not just the last term.

What happened to the "-4" in the last term? It looks like it just disappeared.

You might find it more useful to start by multiplying the last two factors first, since this gives you 81x² - 16. Then multiply that by the x - 6.

Eliz.

 

[Denis]

(x-6)(9x+4)(9x-4)
Using the FOIL method on the first two I came up with
9x^2+4x-54x+24(9x-4)
9x(9x^2+4x-54x+24)
81x^3+36x^2-486x^2-216x
81x^3-450x^2-216x
-4(9x^2+4x-54x+24)
-36x^2-16x+216x+96
-36x^2+200x+96
Then I combined like terms
81x^3-486x^2-16x+96
Correct?

Yes, correct, BUT after 2 offsetting errors!

 

[Guest]

multiplying

Hi Denis,
I warned you I would be moving on to advanced Algebra. Thanks for you help again.

You too Eliz. I greatly appreciate it.