由题意a-b=2,a-c=1/2得,b-c=-(3/2)
则(b-c)^2+3b-3c+9/4=(b-c)^2+3(b-c)+9/4
=-(3/2)^2+[-(3/2)]+9/4
=9/4—3/2+9/4
=3