# Subtracting two complex Numbers

• Sep 13th 2009, 11:07 AM
tsmith
Subtracting two complex Numbers
This is so confusing
(8 + sqrt of -18) - (4 + 3 sqrt of 2i)
• Sep 13th 2009, 11:34 AM
Chris L T521
Quote:

Originally Posted by tsmith
This is soooooo confusing
(8 + sqrt of -18) - (4 + 3 sqrt of 2i)

Is it $(8+\sqrt{-18})-(4+3\sqrt{2}i)$?

If so, note that $\sqrt{-18}=\sqrt{9(-2)}=3\sqrt{-2}=3\sqrt{2}i$

Now simplify $8+3\sqrt{2}i-4-3\sqrt{2}i$ by combining the real and imaginary parts.

Can you continue from here?
• Sep 13th 2009, 11:46 AM
tsmith
So the 3 sqrt 2i will cancel each other out, right? So the answer would be 4?
• Sep 13th 2009, 12:57 PM
VonNemo19
Quote:

Originally Posted by tsmith
So the 3 sqrt 2i will cancel each other out, right? So the answer would be 4?

yes.
• Sep 14th 2009, 05:53 AM
HallsofIvy
That is, assuming you meant $4+ 3i\sqrt{2}$ and not $4+ 3\sqrt{2i}$. Not being able to tell is that "i" is inside or outside the square root is a good reason to always write "i" in front of square roots, not behind them.