Toán Lớp 8: Giải giúp mình mấy bài này nha
a) (-2-7b)×(15-b^2+b^5)
b) (d^2+3cd+9c^2)×(d-3c)
c) x(x+2)×(-2x+3)
d) 1/2x^2×(-x+7)×(-x-7)
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Toán Lớp 8: Giải giúp mình mấy bài này nha
a) (-2-7b)×(15-b^2+b^5)
b) (d^2+3cd+9c^2)×(d-3c)
c) x(x+2)×(-2x+3)
d) 1/2x^2×(-x+7)×(-x-7)
Comments ( 1 )
a)\left( { – 2 – 7b} \right).\left( {15 – {b^2} + {b^5}} \right)\\
= – 30 + 2{b^2} – 2{b^5} – 105b + 7{b^3} – 7{b^6}\\
b)\left( {{d^2} + 3cd + 9{c^2}} \right)\left( {d – 3c} \right)\\
= {d^3} – {\left( {3c} \right)^3}\\
= {d^3} – 27{c^3}\\
c)x\left( {x + 2} \right).\left( { – 2x + 3} \right)\\
= \left( {{x^2} + 2x} \right)\left( { – 2x + 3} \right)\\
= – 2{x^3} + 3{x^2} – 4{x^2} + 6x\\
= – 2{x^3} – {x^2} + 6x\\
d)\dfrac{1}{2}{x^2}\left( { – x + 7} \right)\left( { – x – 7} \right)\\
= \dfrac{1}{2}{x^2}\left( {x – 7} \right)\left( {x + 7} \right)\\
= \dfrac{1}{2}{x^2}\left( {{x^2} – 49} \right)\\
= \dfrac{1}{2}{x^4} – \dfrac{{49}}{2}{x^2}
\end{array}$