Question

1.dùng định nghĩa hai pahn thức bằng nhau để CM phân thức sau bằng nhau

\(\dfrac{x^3-9x}{15-5x}=\dfrac{-x^2-3x}{5}\)

2.dung dinh nghia hai phan thuc bang nhau de tim A

\(\dfrac{4x^2-3x+7}{A}=\dfrac{4x-7}{2x+3}\)

Answer 1

Bài 1:

\(\dfrac{x^3-9x}{15-5x}=-\dfrac{x\left(x^2-9\right)}{5\left(x-3\right)}=\dfrac{-x\left(x-3\right)\left(x+3\right)}{5\left(x-3\right)}=\dfrac{-x\left(x+3\right)}{5}=\dfrac{-x^2-3x}{5}\)

Bài 2:

Sửa đề: \(\dfrac{4x^2-3x-7}{A}=\dfrac{4x-7}{2x+3}\)

\(\Leftrightarrow A=\dfrac{\left(4x^2-3x-7\right)\left(2x+3\right)}{4x-7}\)

\(=\dfrac{4x^2-7x+4x-7}{4x-7}\cdot\left(2x+3\right)\)

\(=\left(x+1\right)\left(2x+3\right)\)