# How do you solve the following limit without a calculator?

1. Nov 27, 2004

### KataKoniK

Hi,

I was wondering, how would one solve the following equation without using a calculator. In other words, algebraically.

lim (x + sqrt(x^2+5x))
x-> -infinity

2. Nov 27, 2004

### arildno

Multiply with the conjugate expression:
$$x+\sqrt{x^{2}+5x}=(x+\sqrt{x^{2}+5x})\frac{x-\sqrt{x^{2}+5x}}{x-\sqrt{x^{2}+5x}}=-\frac{5x}{x-\sqrt{x^{2}+5x}}\to-\frac{5}{2}, x\to\infty$$

3. Nov 27, 2004

### KataKoniK

Thanks a lot. Really appreciate it. I thought you had to do it a certain way because the limit is approaching infinity instead of a number.

4. Nov 27, 2004

### KataKoniK

Didn't notice this, but how does the bottom become 2?

5. Nov 27, 2004

### marlon

Because when calculating the limit to -infinity you need to put the denominator $$x-\sqrt{x^{2}+5x}$$ in factorized form. When doing so you need to get an x² out of the square-root but realize that x is negative so you need to write $$x-(-x)\sqrt{1+\frac{5x}{x^2}}$$. This is just like saying that $$\sqrt{9} = \sqrt{(-3)(-3)} = -3$$. Factoring on you will get that $$x(1+\sqrt{1+\frac{5}{x}})$$ and the x will vanish because of the x you will get in the nominator after completing the exact same procedure there. If you fill in $$- \infty$$ you will get the 2 in the bottom

regards
marlon

6. Nov 27, 2004

Thank you!