/0.3/+/-0.2/
=0.3+0.2
=0.5
/三分之二/—/-二分之一/
=2/3-1/2
=4/6-3/6
=1/6
/-1.75/×/负七分之二/
=1.75×2/7
=(1+3/4)×2/7
=7/4×2/7
=1/2