\(2x=3y\Rightarrow\dfrac{x}{3}=\dfrac{y}{2}\)

Theo tc dãy tỉ số bằng nhau ta có 

\(\dfrac{x}{3}=\dfrac{y}{2}=\dfrac{x-5y}{3-10}=\dfrac{2,1}{-7}=-\dfrac{3}{10}\Leftrightarrow x=-\dfrac{9}{10};y=-\dfrac{3}{5}\)

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Từ E dựng đường thẳng \(Ez\perp DE\Rightarrow\widehat{DEz}=90^o\)

Dx//Ez (cùng vuông góc với DE)

\(\Rightarrow\widehat{FEz}=\widehat{DEF}-\widehat{DEz}=125^o-90^o=35^o\)

\(\Rightarrow\widehat{FEz}+\widehat{EFy}=35^o+145^o=180^o\) Hai góc này ở vị trí là 2 góc trong cùng phía bù nhau => Ez//Fy

=> Dx//Fy (cùng // với Ez)

a) Qua O kẻ Ox song song với m, n

Ta có \(\widehat{DOx}=180^0-\widehat{ODn}=180^0-134^0=46^0\)

Do \(Ox//m\Rightarrow Ox\perp OC\)

\(\Rightarrow\widehat{COD}=\widehat{COx}+\widehat{DOx}=90^0+46^0=136^0\)

b) Do \(\left\{{}\begin{matrix}OC\perp m\\m//n\end{matrix}\right.\Rightarrow n\perp OC\)

Cảm ơn bạn

Ta có: \(\dfrac{3}{11}\cdot\left(\dfrac{8}{3}\cdot\dfrac{7}{4}\right)\)

\(=\dfrac{3}{11}\cdot\left(\dfrac{2}{3}\cdot\dfrac{7}{1}\right)\)

\(=\dfrac{3}{11}\cdot\dfrac{14}{3}\)

\(=\dfrac{1}{11}\cdot\dfrac{14}{1}\)

\(=\dfrac{14}{11}\)

Vậy \(\dfrac{3}{11}\cdot\left(\dfrac{8}{3}\cdot\dfrac{7}{4}\right)=\dfrac{14}{11}\).

lấy máy casio bấm là ra nhé :)

\(\dfrac{308}{17}+\dfrac{15}{23}+\dfrac{22}{17}+\dfrac{3}{19}+\dfrac{8}{23}\)

\(=\left(\dfrac{308}{17}+\dfrac{22}{17}\right)+\left(\dfrac{15}{23}+\dfrac{8}{23}\right)+\dfrac{3}{19}\)

\(=\dfrac{330}{17}+1+\dfrac{3}{19}\)

\(=\dfrac{6644}{323}\)