68×39+39×33-33=68×39+39×33-39×1+6=39×(68+33-1)+6=39×100+6=3900+6=3906
因为39x33前是″十″号,原式=68x39+33ⅹ(39-1)=68x39+33x38=68x(38+1)+33x38=68x38+68+33ⅹ38=101x38+68=3906