Question

1) Tính:

a) (x-2)(x²+3x+4)
b) (x-2)(x-x²+4)
c) (x²-1)(x²+2x)
d) (2x-1)(3x+2)(3-x)
e) (x+3)(x²+3x-5)
f) (xy-2)(x³-2x-6)
g) (5x³-x²+2x-3)(4x²-x+2)

2) tìm x:
a) 3x³-3x=0
b) x²-x+\(\frac{1}{4}=0\)

Answer 1

2)

a) \(3x^3-3x=0\)

\(\Leftrightarrow3x\left(x^2-1\right)=0\)

\(\Leftrightarrow3x\left(x-1\right)\left(x+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}3x=0\\x-1=0\\x+1=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=1\\x=-1\end{matrix}\right.\)

Vậy x=0 ; x=-1 ; x=1

b) \(x^2-x+\dfrac{1}{4}=0\)

\(\Leftrightarrow x^2-2.x.\dfrac{1}{2}+\left(\dfrac{1}{2}\right)^2=0\)

\(\Leftrightarrow\left(x-\dfrac{1}{2}\right)^2=0\)

\(\Leftrightarrow x-\dfrac{1}{2}=0\)

\(\Leftrightarrow x=\dfrac{1}{2}\)

Vậy \(x=\dfrac{1}{2}\)

Answer 2

1)

a) \(\left(x-2\right)\left(x^2+3x+4\right)\)

\(\Leftrightarrow x^3+3x^2+4x-2x^2-6x-8\)

\(\Leftrightarrow x^3+x^2-2x-8\)

b) \(\left(x-2\right)\left(x-x^2+4\right)\)

\(=x^2-x^3+4x-2x+2x^2-8\)

\(=3x^2-x^3+2x-8\)

c) \(\left(x^2-1\right)\left(x^2+2x\right)\)

\(=x^4+2x^3-x^2-2x\)

d) \(\left(2x-1\right)\left(3x+2\right)\left(3-x\right)\)

\(=\left(6x^2+4x-3x-2\right)\left(3-x\right)\)

\(=18x^2+12x-9x-6-6x^3-4x^2+3x^2+2x\)

\(=17x^2+5x-6-6x^3\)