（-13）+（+12）+（-7）+（+38）
=-13+12-7+38
=-1-7+38
=-8+38
=30
1/3-1/2-3/4+2/3
=(1/3+2/3)-(1/2+3/4)
=1-5/4
=-1/4
（-7.4）+2.6+（-0.6）+（-0.5）+5.9
=-7.4+2.6-0.6-0.5+5.9
=(2.6-0.6)+5.9-(7.4+0.5)
=2+5.9-7.9
=7.9-7.9
=0