a²+4b²-a+4b+5/4
=(a²-a+1/4)+(4b²+4b+1)
=(a-1/2)²+(2b+1)²=0
(a-1/2)²>=0,(2b+1)²>=0
a-1/2=0
2b+1=0
a=1/2
b=-1/2
-ab=1/4
-ab的平方根=±1/2