Để  ( a ; b ) ⊂ ( c ; d ) thì  c ≤ a < b ≤ d

Đáp án D

lỗi rồi bạn nhé

\(\dfrac{a}{b}< \dfrac{c}{d}\Rightarrow ad< bc\\ \Rightarrow ad+ab< bc+ab\\ \Rightarrow a\left(b+d\right)< b\left(a+c\right)\)

\(\Rightarrow\)\(\dfrac{a}{b}< \dfrac{a+c}{b+d}\)

Câu trắc nghiệm này kinh thật :D

\(P=\left(1+36abc\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)=\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}+36\left(ab+bc+ca\right)\)

\(P=\left(a+b+c\right)\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)+36\left(ab+bc+ca\right)\)

\(P=\dfrac{a^2+b^2}{ab}+\dfrac{b^2+c^2}{bc}+\dfrac{c^2+a^2}{ca}+3+36\left(ab+bc+ca\right)\)

\(P=\dfrac{\left(a+b\right)^2}{ab}+\dfrac{\left(b+c\right)^2}{bc}+\dfrac{\left(c+a\right)^2}{ca}+36\left(ab+bc+ca\right)-3\)

\(P\ge\dfrac{\left(2a+2b+2c\right)^2}{ab+bc+ca}+36\left(ab+bc+ca\right)-3\)

\(P\ge\dfrac{4}{ab+bc+ca}+36\left(ab+bc+ca\right)-3\)

\(P\ge2\sqrt{\dfrac{144\left(ab+bc+ca\right)}{ab+bc+ca}}-3=21\)

Vậy \(P\ge21\)

a/b = b/c = c/d = (a+b+c)/(b+c+d)

=> (a+b+c/b+c+d)^6054 = (a/b)^6054

Theo đề, ta có:

\(\left\{{}\begin{matrix}a\ge c+d\\b\ge c+d\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a-c\ge d\ge0\\b-d\ge c\ge0\end{matrix}\right.\) 

\(\Rightarrow\left(a-c\right)\left(b-d\right)\ge cd\)

\(\Leftrightarrow ab-bc-ad+cd\ge cd\)

\(\Leftrightarrow\) \(ab\ge ad+bc\left(đpcm\right)\)