# Homework Help: Complex numbers, Homework Question.

1. Oct 1, 2007

### POLISH_Soldier

Express this in terms of j

6j-5j2√-63

I have no idea how to do the ones with square roots, my teacher is lost. Completely and im stuck on this 1 number for like 2 hrs trying to figure it out.

**Note that j2= -1

NOTE ALSO THAT the 2 is a 5jsquare

2. Oct 1, 2007

### symbolipoint

You seem to have written the given expression incorrectly. The question itself seems incredibly simple. Can we assume that for "j", you mean j*j= -1 ? Let me try to continue:

[note, you might really want to see the part below starting with "Where..." ]
6j - 5j2(-63)^(0.5)
6j -5*2*j*j(3*3*7)^(0.5)
6j -5*2*3*(-1)*(7^0.5)
6j + 30*7^(0.5)

Where did you find j2 = -1 ? Is that simply a given condition? "2" is a subscript?
6j - 5j2(-63)^(0.5)
6j + 5*(-1)*3*(7^(0.5))
6j - 15*(7^(0.5))

3. Oct 1, 2007

### HallsofIvy

Oh, those engineers and their jmaginary numbers!

Any normal person would use "i" but those engineers have to use "j"!
I assume you mean 6j-5j^2√-63. If you won't use LaTex at least use ^2 to indicate squares. Yes, J^2= -1 so obviously $6j- 5j^2\sqrt{-3}= 5j\sqrt{63}+ 6j= (5\sqrt{63}+6)j$. What more is there to say?

4. Dec 1, 2008

### kevtimc

It should be $$\sqrt{-63}$$ in your first equation. You confused the heck out of me for a few minutes

If all 3 roots of $az^3+bz^2+cz+d=0$ have negative real part Dec 3, 2015