Ответ:

  1. \(\frac{12\cdot21}{35\cdot15}=\frac{12\cdot3}{5\cdot15}=\frac{12}{25}\).

  2. \(\frac{72\cdot11}{33\cdot30}=\frac{72}{3\cdot30}=\frac{4}{5}\).

  3. \(\frac{25\cdot17\cdot44}{51\cdot8\cdot75}=\frac{1\cdot1\cdot11}{3\cdot3\cdot2}=\frac{11}{18}\).

  4. \(\frac{8\cdot3+8\cdot23}{8\cdot16}=\frac{8(3+23)}{8\cdot16}=\frac{13}{8}\).

  5. \(\frac{17\cdot48}{17\cdot16-9\cdot16}=\frac{17\cdot48}{16(17-9)}=\frac{51}{8}\).

  6. \(\frac{14\cdot5-14\cdot3}{21\cdot9+21\cdot3}=\frac{14(5-3)}{21(9+3)}=\frac{1}{9}\).

Ответ: 1) 12/25; 2) 4/5; 3) 11/18; 4) 13/8; 5) 51/8; 6) 1/9.