# Binomial Expansion Question

1. Dec 29, 2012

### bllnsr

2. Dec 29, 2012

### Dick

3. Dec 29, 2012

### skiller

You got the correct answer somehow, but your working is flawed. Can you see where?

4. Dec 29, 2012

### Dick

Ooops. Yeah, I agree. I somehow just read through the bad part. -1/0 should have been a tip off.

5. Dec 30, 2012

### bllnsr

oh yes -1/0 is -∞ but I made it zero
a/3 +1 = -1/2x
putting x = 0
a/3 +1 = -1/2(0)
a/3 +1 = -1/0
-1/0 is -∞
a/3 +1 = -∞
if a = -3 is the correct answer how to get this value

Last edited: Dec 30, 2012
6. Dec 30, 2012

### skiller

The question states "the coefficient of the term in x is zero".

What do you think this coefficient is?

7. Dec 30, 2012

### bllnsr

somebody told me this general formula
$T_{r+1} = \binom{n}{r}a^n b^r$
will be used to find 'a' and the statement "the coefficient of the term in x is zero" means
that $\binom{n}{r}$ is 0 and what I did previously is wrong.
I have math exam tomorrow and this is the only question that I cannot solve

Last edited: Dec 30, 2012
8. Dec 30, 2012

### Dick

What you did before is almost right. When you get to 1+(2ax/3)+2x=1+(2a/3+2)x the part you want to make 0 is just the coefficient of x, (2a/3+2). Ignore the 1, it doesn't have anything to do with x.

9. Dec 30, 2012

### bllnsr

@Dick
can you please show me last two steps of how to solve it for a

10. Dec 30, 2012

### Dick

Ok, just for you. 2a/3+2=0, 2a/3=(-2) (subtract 2 from both sides), 2a=(-2)*3 (multiply both sides by 3), a=(-2)*3/2=(-3) (divide both sides by 2).

11. Dec 30, 2012

### bllnsr

Thanks

12. Dec 30, 2012

### Dick

You're welcome. Notice no 1/0 appears. If it does that's a pretty sure sign something is wrong.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook