# Pure Math 1 Help & Expanding trinomial

• Jan 10th 2013, 04:41 PM
ThanhPhungLe1112
Pure Math 1 Help & Expanding trinomial
Hi,
I have a couple of homework questions, can anyone help me please?

1) Given that the expansion of (1+ax)^n begins 1+36x+576x^2, find the values of a and n.
2) Determine the coefficient of p^4q^7 in the expansion of (2p-q)(p+q)^10.

3. How to expand the entire trinomial: (10+9x3+8/(3x5) )3

Thank you very much(Hi)
• Jan 10th 2013, 08:14 PM
chiro
Re: Pure Math 1 Help & Expanding trinomial
Hey ThanhPhungLe1112.

Hint for 1: What is the formula for the first three terms of a binomial expansion? Can you get results for a in terms of two of the coefficients?
• Jan 11th 2013, 05:57 PM
ThanhPhungLe1112
Re: Pure Math 1 Help & Expanding trinomial
um...This is what I have for number 1. I'm not sure if it's right

nC0 (1)n(ax)0 + nC1 (1)n-1(ax)1 + nC2 (1)n-2(ax)2

(1)(1)(1) + nC1 (1) (ax) + nC2 (1) (a2x2)

nC1 (ax) = 36x
nC1 (a) = 36

nC2 (a2x2) = 576x2
nC2 (a2) = 576

But then from here on, I'm stuck.
Thanks,
Phung

PS: I know that somehow you have to use the factorials formula, but I don't know how to apply it in this case.
• Jan 11th 2013, 06:09 PM
chiro
Re: Pure Math 1 Help & Expanding trinomial
What is the relationship between nC1 and nC2? Once you have this you will get a simple relationship between n and a that doesn't involve factorials and you can solve for n and a.
• Jan 11th 2013, 06:52 PM
ThanhPhungLe1112
Re: Pure Math 1 Help & Expanding trinomial
is it nC2 = nC1-n ? I'm really confused :(
• Jan 11th 2013, 07:01 PM
chiro
Re: Pure Math 1 Help & Expanding trinomial
Expand out both and simplify and try and get one in terms of a ratio and a difference.
• Jan 14th 2013, 07:36 PM
ThanhPhungLe1112
Re: Pure Math 1 Help & Expanding trinomial
okay, Thanks I got the answer for that problem.
But I have another assigment. So I was supposed to make a powerpoint explaining how to expand trinomials. I made my powerpoint, but when we did peer review in class this morning, some people said that I made a mistake. I can't figure out what I did wrong though so can you please help me by viewing this powerpoint and point out the mistake? I don't think it's calculation error, because I don't think they had times to check calculations so I must have solved it wrong.