a与-2b互为相反数,则a-2b=0
b与3c互为相反数,则b+3c=0
于是
1/4a+1/2b+3c+3/2
=1/4a-1/2b+b+3c+3/2
=1/4*(a-2b)+(b+3c)+3/2
=1/4*0+0+3/2
=3/2