=3/4*a²+1/4*a²+b²+ab-a-2b
=3/4*a²+(a/2+b)²-2(a/2+b)
=3/4*a²+(a/2+b)²-2(a/2+b)+1-1
=3/4*a²+(a/2+b-1)²-1
当a=0,b=1时,前两项=0
所以最小值为-1