Toán Lớp 8: x^2 + (3x – 3)(x + 5) = 25
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Toán Lớp 8: x^2 + (3x – 3)(x + 5) = 25
tìm x:
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x^2 + (3x – 3)(x + 5) = 25
<=>x^2+3x^2+15x-3x-15=25
<=>4x^2+12x-15-25=0
<=>4x^2+12x-40=0
<=>4x^2-8x+20x-40=0
<=>(4x^2-8x)+(20x-40)=0
<=>(x-2)(4x+20)=0
<=>[(x-2=0),(4x+20=0):}
<=>[(x=2),(x=-5):}
Vậy x=2;x=-5
x² + ( 3x – 3 ) ( x + 5 ) = 25
⇔ x² + 3x² + 15x – 3x – 15 = 25
⇔ 4x² + 12x – 15 – 25 = 0
⇔ 4x² + 12x – 40 = 0
⇔ 4x² – 8x + 20x – 40 = 0
⇔ ( 4x² – 8x ) + ( 20x – 40 ) = 0
⇔ 4x ( x – 2 ) + 20 ( x – 2 ) = 0
⇔ ( x – 2 ) ( 4x + 20 ) = 0
⇔ $\left[\begin{matrix} x – 2 = 0 \\ 4x + 20 = 0 \end{matrix}\right.$
⇔ $\left[\begin{matrix} x = 2 \\ x = -5 \end{matrix}\right.$
Vậy x = 2 hoặc x = -5