Es sei w= a+ ib ∈ ℂ mit b ≠0. Es sei z= x1+iy1 die Quadratwurzel von w mit x1>0.
Dann gilt doch
(x1+iy1 )2 = a+bi
x12 +2*x1y1*i -y12 = a+bi
==> x12 - y12 = a und 2x1*y1=b
==> x12 = a + y12 und y1 = b/(2x1)
==> x12 = a + b2 / (4x12)
==> 4x14 = 4ax12 + b2 #
außerdem |w|=√(a2+b2), also b2 = |w|2 - a2
Damit wird # zu
<=> 4x14 = 4ax12 + |w|2 - a2
<=> 4x14 - 4ax12 + a2 = |w|2
<=> ( 2x12 - a ) 2 = |w|2
<=> 2x12 - a = |w| oder 2x12 - a = -|w|
<=> x12 = ( |w| + a )/2 oder x12 = ( a-|w| ) /2
wegen x1>0 also x1 = √ ( ( |w| + a )/2 )