Question

алгебра, алгебра.........​

Answer 1

$\displaystyle\bf\\11)\\\\C_{25} ^{2} =\frac{25!}{2!\cdot(25-2)!} =\frac{25!}{2!\cdot 23! } =\frac{25\cdot 24\cdot23!}{1\cdot 2\cdot23!} =25\cdot12=300\\\\\\12)\\\\5,2(6)=5\frac{26-2}{90} =5\frac{24}{90} =5\frac{4}{15} \\\\\\13)\\\\\frac{5a}{\sqrt{5} +2} =\frac{5a\cdot(\sqrt{5} -2)}{(\sqrt{5} +2)(\sqrt{5} -2)} =\frac{5a\cdot(\sqrt{5} -2)}{(\sqrt{5} )^{2}-2^{2} } =\frac{5a(\sqrt{5} -2)}{5-4} =5a(\sqrt{5} -2)$

$\displaystyle\bf\\14)\\\\\sqrt{7-4\sqrt{3} } =\sqrt{4-4\sqrt{3} +3} =\sqrt{2^{2} -2\cdot 2\cdot\sqrt{3} +(\sqrt{3} )^{2} } =\\\\=\sqrt{(2-\sqrt{x} )^{2} } =|2-\sqrt{3} |=2-\sqrt{3}$