# Math Help - Proving complex numbers

1. ## Proving complex numbers

I need help with this problem, I'm very confused

Prove that for all complex numbers z and w:

1. zw* + z*w is always real

2. zw* - z*w is purely imaginary or zero.

2. Originally Posted by erickvm123
I need help with this problem, I'm very confused

Prove that for all complex numbers z and w:

1. zw* + z*w is always real

2. zw* - z*w is purely imaginary or zero.
if you mean by z* the conjugate here is the answer
let

$z = x+iy , w = u+iv$

$\bar{z} = x -iy , \bar{w} = u-iv$

$z\bar{w} = xu+yv+i(yu-vx)$

$\bar{z} w = xu+vy +i(vx-uy) = xu+vy -i(uy-vx)$

$z\bar{w} + \bar{z} w = xu+yv+i(yu-vx)+xu+vy -i(uy-vx) = 2(xu+yv)$
and this is always real

$z\bar{w} - \bar{z} w = xu+yv+i(yu-vx) - (xu+vy -i(uy-vx)) = 2i(yu-vx)$
and this maybe 0 if yu=vx or imaginary if yu dose not equal vx

and it is known that

$z + \bar{z} = 2Re(z)$ Re is the real part of z

$z - \bar{z} = 2Im(z)$ Im is the imaginary part of z