Homework Help: Compute the limit (with variable in exponent)

1. Nov 12, 2011

PirateFan308

1. The problem statement, all variables and given/known data
Compute the following sequence (show your work)

(-2)n+3n
÷ (-2)n+1+3n+1

3. The attempt at a solution
(-2)n+3n
÷ (-2)n(-2)+3n3n

After this I don't know what to do. I know that if I can prove that something smaller than this for all n's converges to +∞, this will converge to +∞. Also, if I can prove that something bigger that this for all n's converges to 0, this will converge to 0. I'm just not sure how to simplify this equation at all.

2. Nov 12, 2011

gb7nash

Is the problem:

$$lim_{n \to \infty} \frac{(-2)^{n}+3^{n}}{(-2)^{n+1}+3^{n+1}}$$

?

Look at this again.

3. Nov 12, 2011

Staff: Mentor

I'm guess that this is the expression (it's not an equation) whose limit you want to find:
$$\frac{(-2)^n + 3^n}{(-2)^{n+1} + 3^{n+1}}$$

Multiply the numerator by 1/3n+1 and multiply the denominator by the same thing.

4. Nov 12, 2011

PirateFan308

So I get $\frac{(-2)^{n}(1/3)^{n+1}+(1/3)}{(-2/3)^{n+1}+1}$

Since (-2)n → +∞; (1/3)n+1 → 0, then the product of the two will converge to 0 and the top will converge to $\frac{1}{3}$

(-2/3)n+1 → 0 so the bottom will converge to 1

So the whole thing will converge to $\frac{1}{3}$

Is this correct?

5. Nov 12, 2011

Staff: Mentor

Your reasoning is flawed here. If one factor in a product is growing large without bound (i.e., approaching ∞) and the other factor is approaching 0, there is nothing you can say about the product. The indeterminate form [0* ∞] is similar to other indeterminate forms such as [0/0] and [∞/∞].

For example, would you say that 4n * (1/3)n also has a limit of zero?

Instead of writing (-2)n * (1/3)n+1, write these together as a single fraction, and then you can take the limit.
Yes.

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