x2-6x+y2+10y+34=-(4z-1)2
=>x2-6x+9+y2+10y+25+(4z-1)2=0=B
=>(x-3)2+(y+5)2+(4z-1)2=0
với mọi x,y,z ta có :
(x-3)2>=0
(y+5)2>=0
(4z-1)2>=0
=>(x-3)2+(y+5)2+(4z-1)2>=0
hay B>=0
dấu bằng xảy ra khi (x-3)2=0 => x-3=0 =>x=3
=>(y+5)2=0 =>y+5=0 =>y=-5
=>(4z-1)2=0 =>4z-1=0 => z=1/4
Vậy y=-5