Ответ:

  1. \(\frac{12\cdot21}{35\cdot15}=\frac{4\cdot3}{5\cdot5}=\frac{12}{25}\).
  2. \(\frac{72\cdot11}{33\cdot30}=\frac{12\cdot1}{3\cdot5}=\frac45\).
  3. \(\frac{25\cdot17\cdot44}{51\cdot8\cdot75}=\frac{1\cdot1\cdot11}{3\cdot3\cdot2}=\frac{11}{18}\).
  4. \(\frac{8\cdot3+8\cdot23}{3\cdot16}=\frac{8(3+23)}{48}=\frac{8\cdot26}{48}=\frac{13}{3}\).
  5. \(\frac{17\cdot48}{17\cdot16-9\cdot16}=\frac{17\cdot48}{16(17-9)}=\frac{17\cdot48}{16\cdot8}=\frac{51}{8}\).
  6. \(\frac{14\cdot5-14\cdot3}{21\cdot9+21\cdot3}=\frac{14(5-3)}{21(9+3)}=\frac{28}{252}=\frac19\).

Ответ: 1) \(\frac{12}{25}\); 2) \(\frac45\); 3) \(\frac{11}{18}\); 4) \(\frac{13}{3}\); 5) \(\frac{51}{8}\); 6) \(\frac19\).