Toán Lớp 8: a, ( x – y ) ( x^2 + 7xy + 7y^2 )
b Tính giá trị P= (3x -2) (9x^2 + 6x +4 ) tại x= 4/3
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Toán Lớp 8: a, ( x – y ) ( x^2 + 7xy + 7y^2 )
b Tính giá trị P= (3x -2) (9x^2 + 6x +4 ) tại x= 4/3
Comments ( 2 )
a)(x – y)({x^2} + 7xy + 7{y^2})\\
= {x^3} + 7{x^2}y + 7x{y^2} – {x^2}y – 7x{y^2} – 7{y^3}\\
= {x^3} + (7{x^2}y – {x^2}y) + (7x{y^2} – 7x{y^2}) – 7{y^3}\\
= {x^3} + (7{x^2}y – {x^2}y) + (7x{y^2} – 7x{y^2}) – 7{y^3}\\
= {x^3} + 6{x^2}y – 7{y^3}\\
b)\\
P = (3x – 2)(9{x^2} + 6x + 4)\\
P = \left( {3x – 2} \right)\left[ {{{\left( {3x} \right)}^2} + 2.3x + {2^2}} \right]\\
P = {\left( {3x} \right)^2} – {2^2}\\
thay\,\,x = \frac{4}{3}\\
\Rightarrow {\left( {3\frac{4}{3}} \right)^2} – {2^2}\\
\Rightarrow {4^2} – {2^2}\\
\Rightarrow 16 – 4\\
\Rightarrow 12\\
\end{array}\]