-21又3分之2+（+3又4分之1）-（-3分之2）-（+4分之1）=-(21+2/3)+(3+1/4)-(-2/3)-(1/4)=-21-2/3+3+1/4+2/3-1/4=-21+3-2/3+2/3+1/4-1/4=-21+3=-18
