# Homework Help: Multiply infinity by a positive number

1. Oct 31, 2011

### PirateFan308

1. The problem statement, all variables and given/known data
Prove that if limXn = +∞ and limYn>0 then limXnYn=+∞

3. The attempt at a solution
limXnYn = limXnlimYn = (c)(+∞) where c is a positive real number

I know in my head that a positive number multiplied by infinity is positive, but I am unsure how to prove this and we have not yet done this particular example in class.

2. Oct 31, 2011

### Deveno

instead of thinking of ∞ as a number, think of:

$$\lim_{n \to \infty} x_n = \infty$$

meaning, no matter how large a positive real number N we choose, for all large enough n, xn > N.

now, suppose

$$\lim_{n \to \infty} y_n = L > 0$$

for large enough n, we can ensure that yn > L/2 > 0.

can we make xnyn larger than any positive real number M?

(what happens if we pick n so that xnis larger than 2M/L, and yn is larger than _____?)