Câu hỏi của Phạm Thị Hường - Toán lớp 8 - Học toán với OnlineMath

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\(\dfrac{a^3}{\left(b+1\right)\left(c+2\right)}+\dfrac{b+1}{12}+\dfrac{c+2}{18}\ge3\sqrt[3]{\dfrac{a^3\left(b+1\right)\left(c+2\right)}{216\left(b+1\right)\left(c+2\right)}}=\dfrac{a}{2}\)

Tương tự: \(\dfrac{b^3}{\left(c+1\right)\left(a+2\right)}+\dfrac{c+1}{12}+\dfrac{a+2}{18}\ge\dfrac{b}{2}\)

\(\dfrac{c^3}{\left(a+1\right)\left(b+2\right)}+\dfrac{a+1}{12}+\dfrac{b+2}{18}\ge\dfrac{c}{2}\)

Cộng vế:

\(VT+\dfrac{5}{36}\left(a+b+c\right)+\dfrac{7}{12}\ge\dfrac{1}{2}\left(a+b+c\right)\)

\(\Rightarrow VT\ge\dfrac{13}{36}\left(a+b+c\right)-\dfrac{7}{12}\ge\dfrac{13}{36}.3\sqrt[3]{abc}-\dfrac{7}{12}=\dfrac{1}{2}\) (đpcm)

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

Tương tự: \(b+\dfrac{1}{b+1}\ge\dfrac{3}{2}\sqrt{b}\) ; \(c+\dfrac{1}{c+1}\ge\dfrac{3}{2}\sqrt{c}\)

Nhân vế:

\(VT\ge\dfrac{27}{8}\sqrt{abc}\ge\dfrac{27}{8}\) (đpcm)

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

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

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

\(P\ge\dfrac{\left(bc+ca+ab\right)^2}{a\left(b+c\right)+b\left(c+a\right)+c\left(a+b\right)}\) (BĐT B.C.S)

\(=\dfrac{ab+bc+ca}{2}\) \(\ge\dfrac{3\sqrt[3]{abbcca}}{2}=\dfrac{3}{2}\) (do \(abc=1\)).

ĐTXR \(\Leftrightarrow a=b=c=1\)

\(\dfrac{a^3}{\left(b+2\right)\left(c+3\right)}+\dfrac{b+2}{36}+\dfrac{c+3}{48}\ge3\sqrt[3]{\dfrac{a^3\left(b+2\right)\left(c+3\right)}{1728\left(b+2\right)\left(c+3\right)}}=\dfrac{a}{4}\)

Tương tự: \(\dfrac{b^3}{\left(c+2\right)\left(a+3\right)}+\dfrac{c+2}{36}+\dfrac{a+3}{48}\ge\dfrac{b}{4}\)

\(\dfrac{c^3}{\left(a+2\right)\left(b+3\right)}+\dfrac{a+2}{36}+\dfrac{b+3}{48}\ge\dfrac{c}{4}\)

Cộng vế:

\(P+\dfrac{7\left(a+b+c\right)}{144}+\dfrac{17}{48}\ge\dfrac{a+b+c}{4}\)

\(\Rightarrow P\ge\dfrac{29}{144}\left(a+b+c\right)-\dfrac{17}{48}\ge\dfrac{29}{144}.3\sqrt[3]{abc}-\dfrac{17}{48}=\dfrac{1}{4}\)

Dấu "=" xảy ra khi \(a=b=c=1\)

chết đăng nhầm sogy nha

\(\left(1+\dfrac{b}{a}\right)\left(1+\dfrac{c}{b}\right)\left(1+\dfrac{a}{c}\right)=8\)

\(\Leftrightarrow\left(a+b\right)\left(a+c\right)\left(b+c\right)=8abc\)

Áp dụng BĐT Cô-si cho 2 số không âm:

\(\left(a+b\right)\left(a+c\right)\left(b+c\right)\ge2\sqrt{ab}.2\sqrt{bc}.2\sqrt{ac}=8abc\)

Dấu "=" xảy ra <=> a = b = c

Vậy, △ABC là tam giác đều (đpcm)

Áp dụng bất đẳng thức Cô si ta có:

\(VT=\left(1+\dfrac{b}{a}\right)\left(1+\dfrac{c}{b}\right)\left(1+\dfrac{a}{c}\right)\ge2\sqrt{\dfrac{b}{a}}\cdot2\sqrt{\dfrac{c}{b}}\cdot2\sqrt{\dfrac{a}{c}}=8\sqrt{\dfrac{abc}{abc}}=8=VP\)

Dấu "=" xảy ra khi a = b = c

Mà VT = VP => a = b = c

=> tam giác ABC đều

\(\left(1+\dfrac{b}{a}\right)\left(1+\dfrac{c}{b}\right)\left(1+\dfrac{a}{c}\right)=8\)

\(\Leftrightarrow\left(\dfrac{a}{a}+\dfrac{b}{a}\right)\left(\dfrac{b}{b}+\dfrac{c}{b}\right)\left(\dfrac{c}{c}+\dfrac{a}{c}\right)=8\)

\(\Leftrightarrow\dfrac{a+b}{a}.\dfrac{b+c}{b}.\dfrac{c+a}{c}=8\)

\(\Leftrightarrow\dfrac{\left(a+b\right)\left(b+c\right)\left(c+a\right)}{abc}=8\)

\(\Leftrightarrow\left(a+b\right)\left(b+c\right)\left(c+a\right)=8abc\)

Với mọi \(a,b,c>0\) ta có :

+) \(\left(a+b\right)^2\ge4ab\) Dấu bằng xảy ra \(\Leftrightarrow a=b\)

+) \(\left(b+c\right)^2\ge4bc\) Dấu bằng xảy ra \(\Leftrightarrow b=c\)

+) \(\left(c+a\right)^2\ge4ca\) Dấu bằng xảy ra \(\Leftrightarrow c=a\)

\(\Leftrightarrow\left(a+b\right)^2.\left(b+c\right)^2.\left(c+a\right)^2\ge64a^2b^2c^2\)

\(\Leftrightarrow\left(a+b\right)\left(b+c\right)\left(c+a\right)\ge8abc\)

Dấu "=" xảy ra \(\Leftrightarrow a=b=c\Leftrightarrow\Delta ABC\) đều \(\left(đpcm\right)\)