Question

Tìm x biết

1.  \(\left(2x-3\right)^2=\left(x+5\right)^2\)

2.   \(x^2\left(x-1\right)-4x^2+8x-4=0\)

3.   \(x^2+7x+12=0\)

4.   \(x^2+3x-18=0\)

5.    \(x\left(x+6\right)-7x-42=0\)

Answer 1

a) (2x - 3)² = (x + 5)²

=> 4x² - 12x + 9 = x² + 10x + 25

=> 4x² - 12x + 9 - (x² + 10x + 25) = 0

=> 3x² - 22x - 16 = 0

=> 3x² - 24x + 2x - 16 = 0

=> 3x(x - 8) + 2(x - 8) = 0

=> (3x + 2)(x - 8) = 0

=> \(\orbr{\begin{cases}3x+2=0\\x-8=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=-\frac{2}{3}\\x=8\end{cases}}\)

b) x²(x - 1) - 4x² + 8x - 4 = 0

=> x²(x - 1) - (2x  - 2)² = 0

=> x²(x - 1) - [2(x- 1)]² = 0

=> x²(x - 1) - 4(x - 1)² = 0

=> (x - 1)(x² - 4(x - 1) = 0

=> (x - 1)(x² - 4x + 4) = 0

=> (x - 1)(x - 2)² = 0

=> \(\orbr{\begin{cases}x-1=0\\x-2=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=1\\x=2\end{cases}}\)

c) x² + 7x + 12 = 0

=> x² + 3x + 4x + 12 = 0

=> x(x + 3) + 4(x + 3) = 0

=> (x + 4)(x + 3) = 0

=> \(\orbr{\begin{cases}x+4=0\\x+3=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=-4\\x=-3\end{cases}}\)

d) x² + 3x - 18 = 0

=> x² + 6x - 3x - 18 = 0

=> x(x + 6) - 3(x + 6) = 0

=> (x - 3)(x + 6) = 0

=> \(\orbr{\begin{cases}x-3=0\\x+6=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=3\\x=-6\end{cases}}\)

e) x(x + 6) - 7x - 42 = 0

=> x(x + 6) - 7(x + 6) = 0

=> (x - 7)(x + 6) = 0

=> \(\orbr{\begin{cases}x-7=0\\x+6=0\end{cases}}\Rightarrow\orbr{\begin{cases}x=7\\x=-6\end{cases}}\)

Answer 2

1. ( 2x - 3 )² = ( x + 5 )²

<=> ( 2x - 3 )² - ( x + 5 )² = 0

<=> [ ( 2x - 3 ) - ( x + 5 ) ][ ( 2x - 3 ) + ( x + 5 ) ] = 0

<=> ( 2x - 3 - x - 5 )( 2x - 3 + x + 5 ) = 0

<=> ( x - 8 )( 3x + 2 ) = 0

<=> \(\orbr{\begin{cases}x-8=0\\3x+2=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=8\\x=-\frac{2}{3}\end{cases}}\)

2. x²( x - 1 ) - 4x² + 8x - 4 = 0

<=> x²( x - 1 ) - ( 4x² - 8x + 4 ) = 0

<=> x²( x - 1 ) - 4( x² - 2x + 1 ) = 0

<=> x²( x - 1 ) - 4( x - 1 )² = 0

<=> ( x - 1 )[ x² - 4( x - 1 ) ] = 0

<=> ( x - 1 )( x² - 4x + 4 ) = 0

<=> ( x - 1 )( x - 2 )² = 0

<=> \(\orbr{\begin{cases}x-1=0\\x-2=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=1\\x=2\end{cases}}\)

3. x² + 7x + 12 = 0

<=> x² + 3x + 4x + 12 = 0

<=> x( x + 3 ) + 4( x + 3 ) = 0

<=> ( x + 3 )( x + 4 ) = 0

<=> \(\orbr{\begin{cases}x+3=0\\x+4=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=-3\\x=-4\end{cases}}\)

4. x² + 3x - 18 = 0

<=> x² - 3x + 6x - 18 = 0

<=> x( x - 3 ) + 6( x - 3 ) = 0

<=> ( x - 3 )( x + 6 ) = 0

<=> \(\orbr{\begin{cases}x-3=0\\x+6=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=3\\x=-6\end{cases}}\)

5. x( x + 6 ) - 7x - 42 = 0

<=> x( x + 6 ) - 7( x + 6 ) = 0

<=> ( x + 6 )( x - 7 ) = 0

<=> \(\orbr{\begin{cases}x+6=0\\x-7=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=-6\\x=7\end{cases}}\)

Answer 3

a,\(\left(2x-3\right)^2=\left(x+5\right)^2\)

\(\left(2x-3\right)^2-\left(x+5\right)^2=0\)

\(\left(2x-3-x-5\right)\left(2x-3+x+5\right)=0\)

\(\left(x-8\right)\left(3x-2\right)=0\)

\(\Rightarrow\orbr{\begin{cases}x-8=0\\3x-2=0\end{cases}\Rightarrow\orbr{\begin{cases}x=8\\x=-\frac{2}{3}\end{cases}}}\)