Toán Lớp 8: Câu 2: Thực hiện phép tính:
a) 12xy (5x – 7y)
b) (11x + 9)(3x – 2)
c) (18×3 y4 – 9) : 9xy
d) (x3 – 11×2 – x + 11) : (x – 11)
Câu 3: Tìm x biết:
a) 5x(x – 7) – 3(x – 7) = 0
b) x2 + 4x -5 = 0
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Toán Lớp 8: Câu 2: Thực hiện phép tính:
a) 12xy (5x – 7y)
b) (11x + 9)(3x – 2)
c) (18×3 y4 – 9) : 9xy
d) (x3 – 11×2 – x + 11) : (x – 11)
Câu 3: Tìm x biết:
a) 5x(x – 7) – 3(x – 7) = 0
b) x2 + 4x -5 = 0
Comments ( 1 )
2)a)12xy\left( {5x – 7y} \right)\\
= 12xy.5x – 12xy.7y\\
= 60{x^2}y – 84x{y^2}\\
b)\left( {11x + 9} \right)\left( {3x – 2} \right)\\
= 11x.3x – 11x.2 + 9.3x – 9.2\\
= 33{x^2} – 22x + 27x – 18\\
= 33{x^2} + 5x – 18\\
c)\left( {18{x^3}{y^4} – 9} \right):9xy\\
= 2{x^2}{y^3} – \dfrac{1}{{xy}}\\
d)\left( {{x^3} – 11{x^2} – x + 11} \right):\left( {x – 11} \right)\\
= \left( {x – 11} \right)\left( {{x^2} – 1} \right):\left( {x – 11} \right)\\
= {x^2} – 1\\
3)a)5x\left( {x – 7} \right) – 3\left( {x – 7} \right) = 0\\
\Leftrightarrow \left( {x – 7} \right)\left( {5x – 3} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
x – 7 = 0\\
5x – 3 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = 7\\
x = \dfrac{3}{5}
\end{array} \right.\\
Vậy\,x = 7;x = \dfrac{3}{5}\\
b){x^2} + 4x – 5 = 0\\
\Leftrightarrow {x^2} + 5x – x – 5 = 0\\
\Leftrightarrow \left( {x + 5} \right)\left( {x – 1} \right) = 0\\
\Leftrightarrow \left[ \begin{array}{l}
x + 5 = 0\\
x – 1 = 0
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = – 5\\
x = 1
\end{array} \right.\\
Vậy\,x = – 5;x = 1
\end{array}$