A) 17x99=17x(98+1)=17x98+17x1=1666+17=1683
17x99=17x(100-1)=17x100-17x1=1700-17=1683
B) 345x101=345x(100+1)=345x100+345x1=34500+345=34845
345x101=345x(102-1)=345x102-345x1=35190-345=34845
C) 57x11=57x(10+1)=57x10+57x1=570+57=627
57x11=57x(12-1)=57x12-57x1=684-57=627
D) 97x999=97x(998+1)=97x998+97x1=96806+97=96903
97x999=97x(1000-1)=97x1000-97x1=97000-97=96903