【例 1】
(1)12+ 7=19.
(2)-12+(-13)= - (12+13)= -25.
(3)15+( -20)=-(20 - 15) = - 5.
(4)- 9+(+9)=0.
【例2】
(1)2+(-13)+8+(-7)
=(2+8)+[(-13)+(-7)]
=10+(-20)
=-10.
(2)1/3+(-3/4)+(+2/3)+(-1/4)
=[1/3+(+2/3)]+[(-3/4)+(-1/4)]
=1+(-1)
=0.
变式探究
(1)27+(+4)+(+4.5)+(-1)=34.5(元).
即星期三 收盘时,每股是34.5元.
(2)本周内每股最高价:27+(+4)+(+4.5)=35.5(元);
最低价:27+(+4)+(+4.5)+(-1)+(-2.5)+(-4)=28(元).