多项式=3a²b-1/2+1/4ab-1/4ab-7a²b+6a
=-4a²b+6a-1/2
a=1/4,b=-1/4
原式=-4×1/16×（-1/4）+6×1/4-1/2
=1/16+3/2-1/2
=1/16+1
=17/16