z=(a+4i)²/2
=a²-16+8ai/2
=4ai+a2/2-8
|z|²=100=(4a)²+(a²/2-8)2
16a²+(a²)²/4+64-8a²=100
(a²)²/4+8a²-36=0
(a²)²+32a²-144=0
(a²+36)(a²-4)=0
a²=4 ,a²=-36 舍
所以a=±2
z=(a+4i)²/2
=a²-16+8ai/2
=4ai+a2/2-8
|z|²=100=(4a)²+(a²/2-8)2
16a²+(a²)²/4+64-8a²=100
(a²)²/4+8a²-36=0
(a²)²+32a²-144=0
(a²+36)(a²-4)=0
a²=4 ,a²=-36 舍
所以a=±2