# Homework Help: Prove limit of complex function

1. Jul 15, 2009

### complexnumber

1. The problem statement, all variables and given/known data

Prove that lim_{z -> 1 - i} [x + i(2x+y)] = 1 + i

where z = x + iy

2. Relevant equations

Prove using definition of complex limit.

3. The attempt at a solution

Start from |x + i(2x + y) - (1 + i)| < epsilon, need to transform LHS to an expression that includes |(x + iy) - (1 - i)|.

|x + i 2x + i y - 1 - i| < epsilon
|(x + iy) + i 2x - 1 - i| < epsilon

2. Jul 15, 2009

### snipez90

Eh, OK I am assuming you handle these kind of limits the same way you do in one-variable calculus. So basically we need to find bounds for |x-1| and |y+1|. Note

|x + i(2x + y) - (1 + i)| = |x + 2xi - 1 - i + iy| = |x + 2xi -1 - i + (-i) + i + iy| = |(x-1)(1+2i) + i i(y+1)|.

So yeah, you should be able to take it from here.

3. Jul 15, 2009

### complexnumber

Continue from your RHS, I get
\begin{align*} |(x-1)(1+2i) + i (y + 1)| =& \left|(1+2i) [(x-1) + \frac{i(y+1)}{1 + 2i}] \right| \\ =& |1+2i| \left| (x-1) + \frac{i(y+1) (1 - 2i)}{(1 + 2i) (1 - 2i)} \right| \\ =& |1+2i| \left| (x-1) + \frac{i(y+1) + 2(y+1)}{1 + 2i - 2i + 4} \right| \\ =& |1+2i| \left| (x-1) + \frac{i + 2}{5} (y+1) \right| \\ =& \sqrt{5} \left| (x-1) + \frac{i + 2}{5} (y+1) \right| \end{align*}

However, this is still not the same as required expression consisting of $$|(x-1) + i(y+1)|$$. What should I do from here?

4. Jul 15, 2009

### snipez90

Hmm, what I was thinking was that you could apply the triangle inequality to
$$|(x-1)(1+2i) + i(y+1)|$$ (sorry about typo above) so that
$$|(x-1)(1+2i) + i(y+1)| \le \sqrt{5}|x-1| + |y +1|.$$
Then require each of the two terms on the right to be less than $$\varepsilon/2,$$ and then choose delta accordingly.

5. Jul 15, 2009

### complexnumber

Thank you for your reply to both of my threads. I get the idea now; i didn't think of triangle inequality at all.