第一题：0.5+（-1/4）-（-2.75）+1/2

0.5+（-1/4）-（-2.75）+1/2
=0.5-1/4+2.75+1/2
=1/2-1/4+11/4+1/2
=2/4-1/4+11/4+2/4
=14/4
=7/2
（-2/3）+（-1/6）-（-1/4）-1/2
= -2/3-1/6+1/4-1/2
= -8/12-2/12+3/12-6/12
= -13/12