Bạn rút gọn ra bao nhiêu rồi mình làm luôn phần c cho.
mình rút gọn đc \(\frac{9x-18}{\left(x-3\right)\left(x+3\right)}\)
\(A=\frac{9x}{x^2-9}-\frac{3}{x-3}-\frac{3}{x+3}\)
\(=\frac{9x}{\left(x-3\right)\left(x+3\right)}-\frac{3\left(x+3\right)}{\left(x-3\right)\left(x+3\right)}-\frac{3\left(x-3\right)}{\left(x+3\right)\left(x-3\right)}\)
\(=\frac{9x-3x-9-3x+9}{\left(x-3\right)\left(x+3\right)}\)
\(=\frac{3x}{x^2-9}\)
Mk ra vầy
\(A=\frac{9x}{x^2-9}-\frac{3}{x-3}-\frac{3}{x+3}\)
\(A=\frac{9x}{\left(x-3\right)\left(x+3\right)}-\frac{3\left(x+3\right)}{\left(x+3\right)\left(x-3\right)}-\frac{3\left(x-3\right)}{\left(x+3\right)\left(x-3\right)}\)
\(A=\frac{9x-3x-9-3x+9}{\left(x-3\right)\left(x+3\right)}\)
\(A=\frac{3x}{x^2-9}\)
b/ Thay x = - 7 , ta có \(A=\frac{3.\left(-7\right)}{\left(-7\right)^2-9}=\frac{-21}{40}\)
mình rút gọn đc như này \(\frac{9x}{x^2-9}-\frac{3}{x-3}-\frac{3}{x+3}\)
\(=\frac{9x}{\left(x-3\right)\left(x+3\right)}-\frac{3}{x-3}-\frac{3}{x+3}\)
\(=\frac{9x-3\left(x+3\right)+3\left(x-3\right)}{\left(x-3\right)\left(x+3\right)}\)
\(=\frac{9x-3x+9+3x-9}{\left(x-3\right)\left(x+3\right)}\)
\(=\frac{9x-18}{\left(x-3\right)\left(x+3\right)}\)
Dòng thứ 3 phải là
\(\frac{9x-3\left(x+3\right)-3\left(x-3\right)}{\left(x+3\right)\left(x+3\right)}\) chứ
Ở trên là dấu " - " sao xuống bn lại ghi " + "