tìm x:
4 ( x + 3 ) - 3 ( x - 3 ) =21
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`a)(x-2)(x^2+2x+4)-x(x-3)(x+3)=26`
`<=>x^3-2^3-x(x^2-9)=26`
`<=>x^3-8-x^3+9x=26`
`<=>9x-8=26`
`<=>9x=34`
`<=>x=34/9`
`b)(x-3)(x^2+3x+9)-x(x+4)(x-4)=21`
`<=>x^3-3^3-x(x^2-16)=21`
`<=>x^3-27-x^3+16x=21`
`<=>16x-27=21`
`<=>16x=48`
`<=>x=3`
\(\Leftrightarrow\dfrac{6}{16}\cdot2< x< \dfrac{20}{21}\cdot\dfrac{21}{4}=5\)
=>3/4<x<5
hay \(x\in\left\{1;2;3;4\right\}\)
\(\dfrac{4}{x}=\dfrac{y}{21}=\dfrac{28}{49}=\dfrac{28:7}{49:7}=\dfrac{4}{9}\\ Vậy:x=\dfrac{4.9}{4}=9\\ y=\dfrac{4.21}{9}=\dfrac{28}{3}\)
\(\dfrac{x}{2}=\dfrac{3}{y}\\ \Leftrightarrow x.y=2.3=6\\ Vậy:\left[{}\begin{matrix}\left(x;y\right)=\left(1;6\right)=\left(6;1\right)\\\left(x;y\right)=\left(2;3\right)=\left(3;2\right)\end{matrix}\right.\)
a) x-15+124=200
x-15=200-124
x-15=76
x=76+15
x=91
Vậy x=91.
b) 3x-21=36:34
3x-21=32
3x-21=9
3x=9+21
3x=30
x=30:3
x=10
Vậy x=10.
c) x-48:16=21
x-3=21
x=21+3
x=24
Vậy x=24.
4 . (x + 3) - 3 . (x - 3) = 21
4.x + 4.3 - 3.x - 3.3 = 21
4x + 12 - 3x - 9 = 21
4x - 3x + 12 - 9 = 21
x + 12 - 9 = 21
x + 3 = 21
x = 21 : 3
x = 7.
Vậy x = 7.
~~~
\(4\left(x+3\right)-3\left(x-3\right)=21\)
\(\Leftrightarrow4x+12-3x+9=21\)
\(\Leftrightarrow x=21-12-9\)
\(\Leftrightarrow x=0\)
Vậy.....