26.若b、c为正则a=-3,b=10,c=5,a-b-(-c)=-3-10-(-5)=-8
若b、c为负则a=3,b=-10,c=-5,a-b-(-c)=3-(-10)-(-(-5))=8
27.已知|x+2|+|2y-3|=0,则x+2=0,2y-3=0即x=-2,y=3/2
所以|3x-2y|=|3*(-2)-2*(3/2)|=9
28.原式=(1/2010-1/2011)+(1/2011-1/2012)-(1/2010-1/2012)
=1/2010-1/2011+1/2011-1/2012-1/2010+1/2012
=0