20/6 đậu chuyên comeback :>

\(\Rightarrow\) Good luck nhé " Đồng chí " 

sao lại \(\dfrac{5}{5}\) nhỉ :0? 

Độ dài đường chéo BD là : \(20\times\dfrac{5}{5}=20\left(cm\right)\)

Diện tích hình thoi là : \(\dfrac{1}{2}\times AC\times BD=\dfrac{1}{2}\times20\times20=200\left(cm^2\right)\)

\(A=\dfrac{3x^2_1+5x_1x_2+3x^2_2}{5x^2_1x_2+5x_1x^2_2}\\ =\dfrac{3x^2_1+6x_1x_2+3x^2_2-x_1x_2}{5x_1x_2\left(x_1+x_2\right)}\\ =\dfrac{3\left(x_1+x_2\right)^2-x_1x_2}{5x_1x_2\left(x_1+x_2\right)}\)

Áp dụng vi ét : \(\left\{{}\begin{matrix}x_1+x_2=3\\x_1x_2=-7\end{matrix}\right.\)

\(=\dfrac{3.\left(3\right)^2-\left(-7\right)}{5.\left(-7\right).3}=\dfrac{27+7}{-105}=\dfrac{-34}{105}\)

\(y=x^2+1\) hay \(y=x^2-1\)

\(X\%:3+45\%=0,7\\ \Leftrightarrow\dfrac{x}{100}:3+\dfrac{9}{20}=\dfrac{7}{10}\\ \Leftrightarrow\dfrac{x}{100}\times\dfrac{1}{3}=\dfrac{7}{10}-\dfrac{9}{20}\\ \Leftrightarrow\dfrac{x}{300}=\dfrac{14-9}{20}\\ \Leftrightarrow\dfrac{x}{300}=\dfrac{5}{20}\\ \Leftrightarrow x=\dfrac{300\times5}{20}=75\)

Vậy \(x=75\)

\(\dfrac{1}{3}+\dfrac{2}{3}:x-\dfrac{1}{8}=\dfrac{1}{2}\\ \Leftrightarrow\dfrac{2}{3}:x=\dfrac{1}{2}+\dfrac{1}{8}-\dfrac{1}{3}\\ \Leftrightarrow\dfrac{2}{3}:x=\dfrac{12+3-8}{24}\\ \Leftrightarrow\dfrac{2}{3}:x=\dfrac{7}{24}\\ \Leftrightarrow x=\dfrac{2}{3}:\dfrac{7}{24}=\dfrac{2}{3}\times\dfrac{24}{7}=\dfrac{16}{7}\)