Ta có: 

Theo bất đẳng thức Cô - si, ta có: \(\sqrt{\left(a+b\right)\left(a+c\right)}+\sqrt{bc}\le\frac{a+b+a+c}{2}+\frac{b+c}{2}=1\)

\(\Rightarrow\sqrt{a}\left(\sqrt{\left(a+b\right)\left(a+c\right)}+\sqrt{bc}\right)\le\sqrt{a}\)hay \(\sqrt{a^2+abc}+\sqrt{abc}\le\sqrt{a}\)

Tương tự ta có: \(\sqrt{b^2+abc}+\sqrt{abc}\le\sqrt{b}\);\(\sqrt{c^2+abc}+\sqrt{abc}\le\sqrt{c}\)

Mà \(abc\le\left(\frac{a+b+c}{3}\right)^3=\frac{1}{27}\Rightarrow\sqrt{abc}\le\frac{1}{3\sqrt{3}}\)

\(\left(\sqrt{a}+\sqrt{b}+\sqrt{c}\right)^2\le3\left(a+b+c\right)=3\)\(\Leftrightarrow\sqrt{a}+\sqrt{b}+\sqrt{c}\le\sqrt{3}\)

Dấu "=" xảy ra khi và chỉ khi \(a=b=c=\frac{1}{3}\)

a=b=c=1/3

tôi đúng mừ

\(a,b,c\ge0\Rightarrow abc\ge0\Rightarrow\sqrt{a^2+abc}\ge\sqrt{a^2}=a\)

Tương tự:\(\sqrt{b^2+abc}\ge b,\sqrt{c^2+abc}\ge c\)

\(\Rightarrow A\ge a+b+c+0=1\)

Đẳng thức xảy ra \(\Leftrightarrow abc=0,a+b+c=1\)(bạn tự giải tiếp)

\(3\sqrt[3]{abc}\le a+b+c\Rightarrow abc\le\frac{\left(a+b+c\right)^3}{27}=\frac{1}{27}\) (BĐT AM-GM)

\(\sqrt{a^2+abc}=\sqrt{a\left(a+bc\right)}=\frac{2}{3}\sqrt{\frac{9}{4}a\left(a+bc\right)}\le\frac{2}{3}\left(\frac{\frac{9}{4}a+a+bc}{2}\right)\) (BĐT AM-GM)

Tương tự: \(\Rightarrow\)\(A\le\frac{1}{3}\left(\frac{9}{4}\left(a+b+c\right)+a+b+c+ab+bc+ca\right)+9\sqrt{\frac{1}{27}}\)

mà \(ab+bc+ca\le\frac{\left(a+b+c\right)^2}{3}=\frac{1}{3}\)

=>giải được

moi hok lop @ minh . com

mình mới học lớp 6

abc:(a+b+c)=100

aba=(a+b+c)x100

abc=a x100+bx100+cx100

ax100+bx10+c=ax100+bx100+cx100

( đề có vẻ sai )

abc:(a+b+c)=100

aba=(a+b+c)x100

abc=a x100+bx100+cx100

ax100+bx10+c=ax100+bx100+cx100

( đề có vẻ sai ) Nếu bn cảm thấy đúng thì k cho mình nhé!Học Tốt

Thêm ĐK:\(a,b,c\in Z\)

+) Xét \(c=0\)

\(\Leftrightarrow ab.0=2\left(a+b\right).0\) (luôn đúng)

\(\Leftrightarrow a,b\) bất kì

+) Xét \(c\ne0\)

\(\Leftrightarrow ab=2\left(a+b\right)\)

\(\Leftrightarrow2a-ab+2b=0\)

\(\Leftrightarrow a\left(2-b\right)-2\left(2-b\right)+4=0\)

\(\Leftrightarrow\left(a-2\right)\left(b-2\right)=4\)

Lập bảng giải ra \(\left(a;b\right)=\left(3;1\right),\left(4;4\right),\left(1;-2\right),\left(0;0\right)\) và các hoán vị

\(P=\dfrac{a^2+b^2+c^2+2\left(ab+bc+ca\right)+1}{a+b+c-abc}=\dfrac{\left(a+b+c\right)^2+1}{a+b+c-abc}\ge\dfrac{\left(a+b+c\right)^2+1}{a+b+c}\)

\(\Rightarrow P\ge a+b+c+\dfrac{1}{a+b+c}\) (1)

\(P=\dfrac{a^2+b^2+c^2+3\left(ab+bc+ca\right)}{\left(a+b+c\right)\left(ab+bc+ca\right)-abc}=\dfrac{\left(a+b\right)\left(b+c\right)+\left(b+c\right)\left(c+a\right)+\left(a+b\right)\left(c+a\right)}{\left(a+b\right)\left(b+c\right)\left(c+a\right)}\)

\(P=\dfrac{1}{a+b}+\dfrac{1}{b+c}+\dfrac{1}{c+a}=\dfrac{1}{a+b+c}\left(\dfrac{a+b+c}{a+b}+\dfrac{a+b+c}{b+c}+\dfrac{a+c+b}{a+c}\right)\)

\(P=\dfrac{1}{a+b+c}\left(3+\dfrac{a}{b+c}+\dfrac{b}{c+a}+\dfrac{c}{a+b}\right)\ge\dfrac{1}{a+b+c}\left(3+\dfrac{\left(a+b+c\right)^2}{2\left(ab+bc+ca\right)}\right)\)

\(P\ge\dfrac{1}{a+b+c}\left(3+\dfrac{\left(a+b+c\right)^2}{2}\right)=\dfrac{3}{a+b+c}+\dfrac{a+b+c}{2}\)

\(\Rightarrow3P\ge\dfrac{3}{2}\left(a+b+c\right)+\dfrac{9}{a+b+c}\) (2)

Cộng vế (1) và (2):

\(\Rightarrow4P\ge\dfrac{5}{2}\left(a+b+c\right)+\dfrac{10}{a+b+c}\ge2\sqrt{\dfrac{50\left(a+b+c\right)}{2\left(a+b+c\right)}}=10\)

\(\Rightarrow P\ge\dfrac{5}{2}\)

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