Question

помагите пожалуйста срочно ​

Answer 1

Ответ:

$\displaystyle 3)\ \ \frac{21}{\sqrt7}=\frac{21\sqrt7}{7}=3\sqrt7\\\\\\4)\ \ -5\sqrt6+2\sqrt6=-3\sqrt6\\\\-5\sqrt6-2\sqrt6=-7\sqrt6\\\\-5\sqrt6\cdot 2\sqrt6=-10\cdot 6=-60\\\\\dfrac{-5\sqrt6}{2\sqrt6}=-\frac{5}{2}=-2,5\\\\\\5)\ \ -\sqrt{176}+3\sqrt{11}-\sqrt{99}=-4\sqrt{11}+3\sqrt{11}-3\sqrt{11}=-4\sqrt{11}$

$7)\ \ a=\sqrt{13}+5\ \ ,\ \ b=\sqrt{13}-b\\\\a^2-b^2=(\sqrt{13}+5)^2-(\sqrt{13}-5)^2=(38+10\sqrt{13})-(38-10\sqrt5)=20\sqrt5$