\(A=\dfrac{3x^2_1+5x_1x_2+3x^2_2}{5x^2_1x_2+5x_1x^2_2}\\ =\dfrac{3x^2_1+6x_1x_2+3x^2_2-x_1x_2}{5x_1x_2\left(x_1+x_2\right)}\\ =\dfrac{3\left(x_1+x_2\right)^2-x_1x_2}{5x_1x_2\left(x_1+x_2\right)}\)
Áp dụng vi ét : \(\left\{{}\begin{matrix}x_1+x_2=3\\x_1x_2=-7\end{matrix}\right.\)
\(=\dfrac{3.\left(3\right)^2-\left(-7\right)}{5.\left(-7\right).3}=\dfrac{27+7}{-105}=\dfrac{-34}{105}\)
\(A=\dfrac{3\left[\left(x_1+x_2\right)^2-2x_1x_2\right]+5x_1x_2}{5x_1x_2\left(x_1+x_2\right)}\)
\(=\dfrac{3\cdot\left[3^2-2\left(-7\right)\right]+5\cdot\left(-7\right)}{5\cdot\left(-7\right)\cdot3}=-\dfrac{34}{105}\)
