minh tink cho bn bn tink vho minh voi nhe

Ta có :

\(M=\dfrac{9}{1}+\dfrac{8}{2}+\dfrac{7}{3}+................+\dfrac{1}{9}\)

\(M=9+\dfrac{8}{2}+\dfrac{7}{3}+.........+\dfrac{1}{9}\)

\(M=\left(\dfrac{8}{2}+1\right)+\left(\dfrac{7}{3}+1\right)+.........+\left(\dfrac{1}{9}+1\right)+1\)

\(M=\dfrac{10}{2}+\dfrac{10}{3}+.............+\dfrac{10}{9}+\dfrac{10}{10}\)

\(M=10\left(\dfrac{1}{2}+\dfrac{1}{3}+...............+\dfrac{1}{10}\right)\)

Vậy \(\dfrac{M}{N}=\dfrac{10\left(\dfrac{1}{2}+\dfrac{1}{3}+...............+\dfrac{1}{10}\right)}{\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+...............+\dfrac{1}{10}}=10\)

Choose the best answer A, B, C or D to complete the following sentences.

I ………. to Australia yet.

A. went

B. have been

C. will go

D. haven't been

Ta có :

\(\dfrac{1}{c}=\dfrac{1}{2}\left(\dfrac{1}{a}+\dfrac{1}{b}\right)\)

\(\Rightarrow\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{c}:\dfrac{1}{2}\)

\(\Rightarrow\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{c}\cdot\dfrac{2}{1}\)

\(\Rightarrow\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{2}{c}\)

\(\Rightarrow\dfrac{b}{ab}+\dfrac{a}{ab}=\dfrac{2}{c}\)

\(\Rightarrow\dfrac{a+b}{ab}=\dfrac{2}{c}\)

\(\Rightarrow2ab=\left(a+b\right)c\)

\(\Rightarrow ab+ab=ac+bc\)

\(\Rightarrow ac-ab=ab-bc\)

\(\Rightarrow a\left(c-b\right)=b\left(a-c\right)\)

\(\Rightarrow\dfrac{a}{b}=\dfrac{a-c}{c-b}\)

Vậy \(\dfrac{a}{b}=\dfrac{a-c}{c-b}\)

Gọi y là chiều dài của thanh sắt nặng 30 kg

Ta có : \(\dfrac{25}{24}=\dfrac{y}{30}\Rightarrow25\cdot30=24y\Rightarrow y=\dfrac{25\cdot30}{24}=\dfrac{25\cdot5}{4}=\dfrac{125}{4}=31.25\)

Vậy nếu 1 thanh sắt dài 25m và nặng 24kg thì 1 thanh sắt như vậy nặng 30kg thì dài 31.25m

Choose the word whose underlined part is pronounced differently from the others.

A. claimed

B. warned

C. occurred

D. existed