Ответ:

  1. \(\sqrt{12}-\sqrt3+\sqrt{27}=2\sqrt3-\sqrt3+3\sqrt3=4\sqrt3\).
  2. \(\sqrt{45}-\sqrt{20}+\sqrt{125}=3\sqrt5-2\sqrt5+5\sqrt5=6\sqrt5\).
  3. \(2\sqrt3+\sqrt{27}-\sqrt{75}=2\sqrt3+3\sqrt3-5\sqrt3=0\).
  4. \((2\sqrt3-1)^2=12-4\sqrt3+1=13-4\sqrt3\).
  5. \((\sqrt7-4)(\sqrt7+4)=7-16=-9\).
  6. \((\sqrt6-\sqrt5)(\sqrt6+\sqrt5)=6-5=1\).