（-1/7）+( -2/35 )-(-2/5) =-5/35-2/35+14/35 =-7/35 =-1/5 1/2+(-2/3)-(-4/5)+(-1/2) =1/2-1/2-2/3+4/5 =-2/3+4/5 =-10/15+12/15 =2/15
