Question

Решите номер 7 и 8 , желательно и 9

Answer 1

$7a); ; frac{sqrt7}{sqrt7-sqrt5}-frac{sqrt5}{sqrt7+sqrt5}=frac{sqrt7(sqrt7+sqrt5)-sqrt5(sqrt7-sqrt5)}{(sqrt7-sqrt5)(sqrt7+sqrt5)}=\\=frac{7+sqrt{35}-sqrt{35}+5}{7-5}= frac{12}{2}=6 \\7b); ; frac{sqrt2}{sqrt3-sqrt2}+frac{sqrt3}{sqrt3+sqrt2}=frac{sqrt2(sqrt3+sqrt2)+sqrt3(sqrt3-sqrt2)}{(sqrt3-sqrt2)(sqrt3+sqrt2)}=\\=frac{sqrt6+2+3-sqrt6}{3-2}=frac{5}{1}=5\\8); ; (sqrt{15}+sqrt{21})(sqrt{21}-sqrt{15})=21-15=6\\9); ; frac{(6sqrt{13})^2}{39}=frac{6^2cdot 13}{3cdot 13}= frac{36}{3}=12$