(51+34/35)÷17 =(51+34/35)*1/17 =51*1/17 +34/35*1/17 =3+2/35=3又2/2519.9+19.9*99 =19.9*(1+99)=19.9*100=1990999*1999=(1000-1)*1999=1999000-1999=1999000-(2000-1)=1999000-2000+1=1997000+1=1997001