Giải bài 2 trang 28 SGK Toán 8 tập 1 - Cánh diều

Đề bài

Thực hiện phép tính:

\(a) - \dfrac{1}{3}{a^2}b\left( { - 6{\rm{a}}{b^2} - 3{\rm{a}} + 9{b^3}} \right)\)

\(b)\left( {{a^2} + {b^2}} \right)\left( {{a^4} - {a^2}{b^2} + {b^4}} \right)\)

\(c)\left( { - 5{{\rm{x}}^3}{y^3}z} \right):\left( {\dfrac{{15}}{2}x{y^2}z} \right)\)

\(d)\left( {8{{\rm{x}}^4}{y^2} - 10{{\rm{x}}^2}{y^4} + 12{{\rm{x}}^3}{y^5}} \right):\left( { - 2{{\rm{x}}^2}{y^2}} \right)\)

Lời giải chi tiết

\(\begin{array}{l}a) - \dfrac{1}{3}{a^2}b\left( { - 6{\rm{a}}{b^2} - 3{\rm{a}} + 9{b^3}} \right)\\ = \left( { - \dfrac{1}{3}{a^2}b} \right).\left( { - 6{\rm{a}}{b^2}} \right) + \left( { - \dfrac{1}{3}{a^2}b} \right).\left( { - 3{\rm{a}}} \right) + \left( { - \dfrac{1}{3}{a^2}b} \right).\left( {9{b^3}} \right)\\ = 2{{\rm{a}}^3}{b^3} + {a^3}b - 3{\rm{a^2}}{b^4}\end{array}\)

\(b)\left( {{a^2} + {b^2}} \right)\left( {{a^4} - {a^2}{b^2} + {b^4}} \right) = {\left( {{a^2}} \right)^3} + {\left( {{b^2}} \right)^3} = {a^6} + {b^6}\)

\(\begin{array}{l}c)\left( { - 5{{\rm{x}}^3}{y^2}z} \right):\left( {\dfrac{{15}}{2}x{y^2}z} \right)\\ = \left( { - 5:\dfrac{{15}}{2}} \right).\left( {{x^3}:x} \right).\left( {{y^2}:{y^2}} \right).\left( {z:z} \right) = \dfrac{{ - 2}}{3}{x^2}\end{array}\)

\(\begin{array}{l}d)\left( {8{{\rm{x}}^4}{y^2} - 10{{\rm{x}}^2}{y^4} + 12{{\rm{x}}^3}{y^5}} \right):\left( { - 2{{\rm{x}}^2}{y^2}} \right)\\ = \left[ {\left( {8{{\rm{x}}^4}{y^2}} \right):\left( { - 2{{\rm{x}}^2}{y^2}} \right)} \right] + \left[ {\left( { - 10{x^2}{y^4}} \right):\left( { - 2{{\rm{x}}^2}{y^2}} \right)} \right] + \left[ {\left( {12{{\rm{x}}^3}{y^5}} \right):\left( { - 2{{\rm{x}}^2}{y^2}} \right)} \right]\\ =  - 4{{\rm{x}}^2} + 5{y^2} - 6{\rm{x}}{y^3}\end{array}\)