判断下列说法是否正确,正确的在括号内画“√”，错误的画“×”。
1. 当$a≥0,b≥0$时,$\sqrt {a}\cdot \sqrt {b}=\sqrt {ab}$. ()
2. 当$a≥0,b≥0$时,$\sqrt {ab}=\sqrt {a}\cdot \sqrt {b}$. ()
3. $\frac {\sqrt {a}}{\sqrt {b}}=\sqrt {\frac {a}{b}}$成立的条件是:$a≥0,b＞0$. ()
4. $\sqrt {\frac {a}{b}}=\frac {\sqrt {a}}{\sqrt {b}}$成立的条件是:$a≥0,b＞0$. ()
5. $\sqrt {(-4)×(-9)}=\sqrt {-4}×\sqrt {-9}$. ()
6. $\sqrt {\frac {-9}{-25}}=\sqrt {\frac {9}{25}}=\frac {\sqrt {9}}{\sqrt {25}}=\frac {3}{5}$. ()
7. $\sqrt {0.5}$是最简二次根式. ()
8. $\sqrt {a^{2}+b^{2}}$是最简二次根式. ()
9. $\sqrt {8a^{2}}=4a(a＞0)$. ()
10. $a\sqrt {-\frac {1}{a}}=\sqrt {-a}$. ()

1. √；2. √；3. √；4. √；5. ×；6. √；7. ×；8. √；9. ×；10. ×

1. 二次根式$\sqrt {(-2)^{2}×6}$的计算结果是 ()
A. $2\sqrt {6}$
B. $-2\sqrt {6}$
C. 6
D. 12

A

2. 若$\frac {\sqrt {a^{2}}}{\sqrt {b^{2}}}=-\frac {a}{b}$,则 ()
A. $a＜0,b＜0$
B. $a＞0,b＞0$
C. $ab＞0$
D. $ab≤0$且$b≠0$

D