Bài 27:
a. $4=2.2=2.\sqrt{4}>2.\sqrt{3}$
b. $-\sqrt{5}< -\sqrt{4}=-2$
Bài 28:
a.
\((\sqrt{2}+\sqrt{3})^2=5+2\sqrt{6}=5+2\sqrt{\frac{24}{4}}< 5+2\sqrt{\frac{25}{4}}=5+2.\frac{5}{2}=10\)
$\Rightarrow \sqrt{2}+\sqrt{3}< \sqrt{10}$
b.
\((\sqrt{3}+2)^2=7+4\sqrt{3}\)
\((\sqrt{2}+\sqrt{6})^2=8+4\sqrt{3}\)
Mà $7+4\sqrt{3}< 8+4\sqrt{3}$
$\Rightarrow (\sqrt{3}+2)^2< (\sqrt{2}+\sqrt{6})^2$
$\Rightarrow \sqrt{3}+2< \sqrt{2}+\sqrt{6}$
c.
$\sqrt{15}.\sqrt{17}=\sqrt{15.17}=\sqrt{(16-1)(16+1)}=\sqrt{16^2-1}$
$<\sqrt{16^2}=16$
d.
\((\sqrt{15}+\sqrt{17})^2=32+2\sqrt{15.17}< 32+2.16=64\) (theo kq phần c)
$\Rightarrow \sqrt{15}+\sqrt{17}< \sqrt{64}=8$
giải hộ mình vs ạ mình cần gấp ạ