a²+b²-4a-2b+5=0
(a²-4a+4)+(b²-2b+1)=0
(a-2)²=(a²-4a+4) and (b-1)²= (b²-2b+1)
(a-2)²+(b-1)²=0(第三步)
(a-2)² =0 and (b-1)²=0
a=2,b=1
a²+b²-4a-2b+5=0
(a²-4a+4)+(b²-2b+1)=0
(a-2)²=(a²-4a+4) and (b-1)²= (b²-2b+1)
(a-2)²+(b-1)²=0(第三步)
(a-2)² =0 and (b-1)²=0
a=2,b=1