Toán Lớp 8: Tính giá trị biểu thức A=\dfrac{3x^2-x}{9x^2-6x+1}A=
9x
2
−6x+1
3x
2
−x
tại x=7x=7.
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Toán Lớp 8: Tính giá trị biểu thức A=\dfrac{3x^2-x}{9x^2-6x+1}A=
9x
2
−6x+1
3x
2
−x
tại x=7x=7.
Comments ( 1 )
$\text{A=$\dfrac{3x²-x}{9x²-6x+1}$}$
$\text{A=$\dfrac{3x²-x)}{(3x)²-2.3x.1+1²}$}$
$\text{A=$\dfrac{x(3x-1)}{(3x-1)²}$}$
$\text{A=$\dfrac{x}{3x-1}$}$
$\text{Thay x=7 vào A ta có}$
$\text{A=$\dfrac{x}{3x-1}$}$
$\text{⇔A=$\dfrac{7}{3×7-1}$}$
$\text{A=$\dfrac{7}{21-1}$}$
$\text{A=$\dfrac{7}{20}$}$