# I'm so lost! Please help.

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• Nov 4th 2008, 08:49 PM
KeiaBlake
I'm so lost! Please help.
Okay the paper says multiply by the conjuate

2+i
____ I think I multiply top and bottom by 4+2i but I'm not sure if I do that
4 +2i or if Im suppose to change the middle sign first or what..? Any advice?
• Nov 4th 2008, 10:44 PM
mr fantastic
Quote:

Originally Posted by KeiaBlake
Okay the paper says multiply by the conjuate

2+i
____ I think I multiply top and bottom by 4+2i but I'm not sure if I do that
4 +2i or if Im suppose to change the middle sign first or what..? Any advice?

You multiply top and bottom by the complex conjugate of the denominator: 4 - 2i.
• Nov 4th 2008, 10:48 PM
earboth
Quote:

Originally Posted by KeiaBlake
Okay the paper says multiply by the conjuate

2+i
____ I think I multiply top and bottom by 4+2i but I'm not sure if I do that
4 +2i or if Im suppose to change the middle sign first or what..? Any advice?

The conjugate of 4+2i is 4-2i.

\$\displaystyle \dfrac{2+i}{4+2i}=\dfrac{(2+i)(4-2i)}{(4+2i)(4-2i)} = \dfrac{8-4i+4i-(-2)}{16-(-4)}=\dfrac{10}{20}=\dfrac12\$