Ta có:

 \(\frac{1}{1+a}=2-\frac{1}{1+b}-\frac{1}{1+c}=\left(1-\frac{1}{1+b}\right)+\left(1-\frac{1}{1+c}\right)\ge\frac{b}{1+b}+\frac{c}{1+c}\ge2\sqrt{\frac{bc}{\left(1+b\right)\left(1+c\right)}}\)

Tương tự:

\(\frac{1}{1+b}\ge2\sqrt{\frac{ac}{\left(1+a\right)\left(1+c\right)}}\)

\(\frac{1}{1+c}\ge2\sqrt{\frac{ab}{\left(1+a\right)\left(1+b\right)}}\)

=> \(\frac{1}{1+a}.\frac{1}{1+b}.\frac{1}{1+c}\ge\frac{8abc}{\left(1+a\right)\left(1+b\right)\left(1+c\right)}\)

=> \(abc\le\frac{1}{8}\)

"=" xảy ra <=> a = b = c = 1/2

Vậy max P = abc = 1/8 đạt tại a = b = c =1/2

\(PT\Leftrightarrow x^2+xy-669xy-669y^2=2019\)

\(\Leftrightarrow x\left(x+y\right)-669y\left(x+y\right)=2019\)

\(\Leftrightarrow\left(x+y\right)\left(x-669y\right)=2019\)

xét TH ra bạn

99999

a) \(\sqrt{2+\sqrt{3}}.\sqrt{2-\sqrt{3}}=\sqrt{\left(2+\sqrt{3}\right)\left(2-\sqrt{3}\right)}=\sqrt{4-3}=\sqrt{1}=1\)

b)

Đặt \(B=\sqrt{4+\sqrt{7}}-\sqrt{4-\sqrt{7}}\)

\(B^2=4+\sqrt{7}-2\sqrt{\left(4+\sqrt{7}\right)\left(4-\sqrt{7}\right)}+4-\sqrt{7}\)

\(=8-2\sqrt{16-7}=8-2\sqrt{9}=8-2.3=8-6=2\)

\(\Rightarrow B=\sqrt{2}\)

a) Đặt \(A=\sqrt{5-2\sqrt{6}}+\sqrt{5+2\sqrt{6}}\)

\(A^2=5-2\sqrt{6}+2\sqrt{\left(5-2\sqrt{6}\right)\left(5+2\sqrt{6}\right)}+5+2\sqrt{6}\)

\(=10+2\sqrt{25-4.6}=10+2\sqrt{1}=10+2=12\)

\(\Rightarrow A=\sqrt{12}\)

b)\(\frac{\sqrt{10}-\sqrt{2}}{\sqrt{5}-1}+\frac{2-\sqrt{2}}{\sqrt{2}-1}=\frac{\sqrt{2}.\sqrt{5}-\sqrt{2}}{\sqrt{5}-1}+\frac{\sqrt{2}.\sqrt{2}-\sqrt{2}}{\sqrt{2}-1}\)

\(=\frac{\sqrt{2}\left(\sqrt{5}-1\right)}{\sqrt{5}-1}+\frac{\sqrt{2}\left(\sqrt{2}-1\right)}{\sqrt{2}-1}=\sqrt{2}+\sqrt{2}=2\sqrt{2}\)

a)

\(\sqrt{\left(3-2\sqrt{2}\right)^2}-\sqrt{\left(\sqrt{8}-4\right)^2}=3-2\sqrt{2}-4+\sqrt{8}\)

\(=3-2\sqrt{2}-4+2\sqrt{2}=3-4=-1\)

b)

\(\frac{2}{\sqrt{3}-1}-\frac{2}{\sqrt{3}+1}=\frac{2\left(\sqrt{3}+1\right)-2\left(\sqrt{3}-1\right)}{3-1}\)

\(=\frac{2\left(\sqrt{3}+1-\sqrt{3}+1\right)}{2}=\sqrt{3}+1-\sqrt{3}+1=1+1=2\)