Bài 51 trang 24 SGK Toán 8 tập 1

Phân tích các đa thức sau thành nhân tử:
a) \({x^3}-{\rm{ }}2{x^2} + {\rm{ }}x\);                           

b) \(2{x^2} + {\rm{ }}4x{\rm{ }} + {\rm{ }}2{\rm{ }}-{\rm{ }}2{y^2}\);

c) \(2xy{\rm{ }}-{\rm{ }}{x^2}-{\rm{ }}{y^2} + {\rm{ }}16\).

Lời giải

a) \({x^3}-{\rm{ }}2{x^2} + {\rm{ }}x{\rm{ }}\)

\(= {\rm{ }}x({x^2}-{\rm{ }}2x{\rm{ }} + {\rm{ }}1){\rm{ }}\)

\( = x\left( {{x^2} - 2x + {1^2}} \right)\)

\( = {\rm{ }}x{\left( {x{\rm{ }}-{\rm{ }}1} \right)^2}\)

b) \(2{x^2} + {\rm{ }}4x{\rm{ }} + {\rm{ }}2{\rm{ }}-{\rm{ }}2{y^2} \)

\(= {\rm{ }}2[({x^2} + {\rm{ }}2x{\rm{ }} + {\rm{ }}1){\rm{ }}-{\rm{ }}{y^2}]\)

\(= {\rm{ }}2[{\left( {x{\rm{ }} + {\rm{ }}1} \right)^2}-{\rm{ }}{y^2}]\)

\( = {\rm{ }}2\left( {x{\rm{ }} + {\rm{ }}1{\rm{ }}-{\rm{ }}y} \right)\left( {x{\rm{ }} + {\rm{ }}1{\rm{ }} + {\rm{ }}y} \right)\)

c) \(2xy{\rm{ }}-{\rm{ }}{x^2}-{\rm{ }}{y^2} + {\rm{ }}16{\rm{ }} \)

\(= {\rm{ }}16{\rm{ }}-{\rm{ }}({x^2}-{\rm{ }}2xy{\rm{ }} + {\rm{ }}{y^2}){\rm{ }}\)

\(= {\rm{ }}{4^2}-{\rm{ }}{\left( {x{\rm{ }}-{\rm{ }}y} \right)^2}\)

\( = \left[ {4 - \left( {x - y} \right)} \right].\left[ {4 + \left( {x - y} \right)} \right]\)

\(= (4 - x + y)(4 + x - y)\)


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