\(Q\le\sqrt{3\left(a+b+b+c+c+a\right)}=\sqrt{6\left(a+b+c\right)}\le\sqrt{6.\sqrt{3\left(a^2+b^2+c^2\right)}}=\sqrt{6\sqrt{3}}\)

Dấu "=" xảy ra khi \(a=b=c=\dfrac{1}{\sqrt{3}}\)

Lại có:

\(a^2+b^2+c^2\le1\Rightarrow0\le a;b;c\le1\)

\(\Leftrightarrow a\left(a-1\right)+b\left(b-1\right)+c\left(c-1\right)\le0\)

\(\Leftrightarrow a+b+c\ge a^2+b^2+c^2=1\)

Do đó:

\(Q^2=2\left(a+b+c\right)+2\sqrt{a^2+ab+bc+ca}+2\sqrt{b^2+ab+bc+ca}+2\sqrt{c^2+ab+bc+ca}\)

\(Q^2\ge2\left(a+b+c\right)+2\sqrt{a^2}+2\sqrt{b^2}+2\sqrt{c^2}\)

\(Q^2\ge4\left(a+b+c\right)\ge4\)

\(\Rightarrow Q\ge2\)

Dấu "=" xảy ra khi \(\left(a;b;c\right)=\left(0;0;1\right)\) và hoán vị

hàng đầu tiên tìm MaxQ áp dụng bđt nào thế thầy?

Đề thi học kỳ 1 trường Ams

**Min

Từ \(a^2+b^2+c^2=1\Rightarrow a^2\le1;b^2\le1;c^2\le1\)

\(\Rightarrow a\le1;b\le1;c\le1\Rightarrow a^2\le a;b^2\le b;c^2\le c\)

Khi đó:

\(\sqrt{a+b^2}\ge\sqrt{a^2+b^2};\sqrt{b+c^2}\ge\sqrt{b^2+c^2};\sqrt{c+a^2}\ge\sqrt{c^2+a^2}\)

\(\Rightarrow P\ge\sqrt{a^2+b^2}+\sqrt{b^2+c^2}+\sqrt{c^2+a^2}\)

\(\Rightarrow P\ge\sqrt{1-c^2}+\sqrt{1-a^2}+\sqrt{1-b^2}\)

Ta có:

\(\sqrt{1-c^2}\ge1-c^2\Leftrightarrow1-c^2\ge1-2c^2+c^4\Leftrightarrow c^2\left(1-c^2\right)\ge0\left(true!!!\right)\)

Tương tự cộng lại:

\(P\ge3-\left(a^2+b^2+c^2\right)=2\)

dấu "=" xảy ra tại \(a=b=0;c=1\) and hoán vị.

**Max

Có BĐT phụ sau:\(\sqrt{a}+\sqrt{b}+\sqrt{c}\le\sqrt{3\left(a+b+c\right)}\left(ezprove\right)\)

Áp dụng:

\(\sqrt{a+b^2}+\sqrt{b+c^2}+\sqrt{c+a^2}\)

\(\le\sqrt{3\left(a+b+c+a^2+b^2+c^2\right)}\)

\(=\sqrt{3\left(a+b+c\right)+3}\)

\(\le\sqrt{3\left(\sqrt{3\left(a^2+b^2+c^2\right)}+3\right)}=\sqrt{3\cdot\sqrt{3}+3}\)

Dấu "=" xảy ra tại \(a=b=c=\pm\frac{1}{\sqrt{3}}\)

bn sử dụng bất đẳng thức cô si đi

Nguyễn Đại Nghĩa,bác nói cụ thể hơn được ko :v

https://h.vn/hoi-dap/question/702421.html

https://h.vn/hoi-dap/question/702421.html

https://h.vn/hoi-dap/question/702421.html

xin lỗi mk nhầm bài

Áp dụng BĐT Bunhiacopxki ta có: 

\(\left(\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a}\right)^2\le\left(1+1+1\right)\left(a+b+b+c+c+a\right)\)

\(=3\left(2a+2b+2c\right)=3.2\left(a+b+c\right)=6.2021=12126\)

\(\Rightarrow\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a}\le\sqrt{12126}\)

Dấu ''='' xảy ra khi \(a=b=c=\dfrac{2021}{3}\)

\(P=\sqrt{a+b}+\sqrt{b+c}\sqrt{c+a}\)

Aps dụng Bunhia-cốpxki : \(P^2=\left(\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a}\right)\le\left(1+1+1\right)\left(a+b+b+c+c+a\right)\)

\(=6\left(a+b+c\right)\)

\(=6.2021=12126\Leftrightarrow P=\sqrt{12126}\)

Vậy \(Max\left(P\right)=\sqrt{12126}\Leftrightarrow a=b=c=\dfrac{2021}{3}\)

(Refer ;-;)

\(\sqrt{2a^2+ab+2b^2}=\sqrt{\dfrac{3}{2}\left(a^2+b^2\right)+\dfrac{1}{2}\left(a+b\right)^2}\ge\sqrt{\dfrac{3}{4}\left(a+b\right)^2+\dfrac{1}{2}\left(a+b\right)^2}=\dfrac{\sqrt{5}}{2}\left(a+b\right)\)

Tương tự:

\(\sqrt{2b^2+bc+2c^2}\ge\dfrac{\sqrt{5}}{2}\left(b+c\right)\) ; \(\sqrt{2c^2+ca+2a^2}\ge\dfrac{\sqrt{5}}{2}\left(c+a\right)\)

Cộng vế với vế:

\(P\ge\sqrt{5}\left(a+b+c\right)\ge\dfrac{\sqrt{5}}{3}\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)^3=\dfrac{\sqrt{5}}{3}\)

Dấu "=" xảy ra khi \(a=b=c=\dfrac{1}{9}\)