Câu 1 : Tìm x , biết :
a ) 3x ( 12x – 4 ) – 9x ( 4x – 3 ) = 30
b ) x ( 5x – 2x ) + 2x ( x – 1 ) = 15
Câu 1 : Tìm x , biết : a ) 3x ( 12x – 4 ) – 9x ( 4x – 3 ) = 30 b ) x ( 5x – 2x ) + 2x ( x – 1 ) = 15
By Audrey
By Audrey
Câu 1 : Tìm x , biết :
a ) 3x ( 12x – 4 ) – 9x ( 4x – 3 ) = 30
b ) x ( 5x – 2x ) + 2x ( x – 1 ) = 15
a, 3x(12x-4)-9x(4x-3)=30 b
36x²-12x-36x²+27x=30 x(5-2x)+2x(x-1)=15
-12x+27x=30 5x-2x²+2x²-2x=15
15x=30 3x = 15
x=30/15=2 x=15/3=5
b ko được khả thi cho lắm 5x mình chỉnh thành 5
Đáp án:
$a) x = 2$
$b)$ `x ∈ {`$\dfrac{1 +2\sqrt{19}}{5}; \dfrac{1 -2\sqrt{19}}{5}$`}`
Giải thích các bước giải:
$a)$ `3x.(12x -4) -9x.(4x -3) = 30`
`⇔ 36x² -12x -36x² +27x = 30`
`⇔ 15x = 30`
`⇔ x = 30 :15`
`⇔ x = 2`
Vậy `x = 2`
$b)$ `x.(5x -2x) +2x.(x -1) = 15`
`⇔ 5x² -2x² +2x² -2x = 15`
`⇔ 5x² -2x -15 = 0`
`⇔ 5.(x² -(2x/5) -3) = 0`
`⇔ 5.[(x)² -2.x.1/5 +(1/5)² -(1/5)² -3] = 0`
`⇔ 5.[(x -1/5)² -76/25] = 0`
$⇔ 5.(x -1/5 -\dfrac{\sqrt{76}}{5}).(x -1/5 +\dfrac{\sqrt{76}}{5}) = 0$
$⇔ \left[ \begin{array}{l}x-1/5 -\dfrac{\sqrt{76}}{5}=0\\x-1/5 +\dfrac{\sqrt{76}}{5}=0\end{array} \right. ⇔ \left[ \begin{array}{l}x=\dfrac{1 +2\sqrt{19}}{5}\\x=\dfrac{1 -2\sqrt{19}}{5}\end{array} \right.$
Vậy `x ∈ {`$\dfrac{1 +2\sqrt{19}}{5}; \dfrac{1 -2\sqrt{19}}{5}$`}`