2a²+2b²-a-b=-1/4
a²-1/2a+1/16+b²-1/2b+1/16=0
(a-1/4)²+(b-1/4)²=0
a-1/4=0 b-1/4=0
∴a=1/4 b=1/4
∴(a-b)×c
=(1/4-1/4)×c
=0×c
=0