Question

Answer 1

\(A=\dfrac{1}{x_1}+\dfrac{1}{x_2}-3x_1x_2=\dfrac{x_1+x_2}{x_1x_2}-3x_1x_2\)

\(=\left(-\dfrac{5}{2}\right):\dfrac{-7}{2}-3\cdot\dfrac{-7}{2}=\dfrac{5}{7}+\dfrac{21}{2}=\dfrac{10+147}{14}=\dfrac{157}{14}\)

Answer 2

Ptr có:`\Delta=5^2-4.2.(-7)=81 > 0`

`=>` Ptr có `2` `n_o` pb

`=>` Áp dụng Viét có:`{(x_1+x_2=[-b]/a=-5/2),(x_1.x_2=c/a=-7/2):}`

Ta có:`A=1/[x_1]+1/[x_2]-3x_1.x_2`

`<=>A=[x_1+x_2-3(x_1.x_2)^2]/[x_1.x_2]`

`<=>A=[-5/2-3(-7/2)^2]/[-7/2]=157/14`