Tìm x biết a) x - 65 =38 -58 b) 4x -15=-39 c) 450 : [41 - (2 x -5 )]= `3^2` x 5

$a)$ $x\:-\:65\:=38\:-58\:$

$⇒x-65=-20$

$⇒x=−20+65$

$⇒x=45$

$\text{Vậy x = 45 }$

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$b)$ $4x-15=-39$

$⇒4x=-39+15$

$⇒4x=-24$

$⇒x=-24:4$

$⇒x=-6$

$\text{Vậy x = -6}$

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$c)$ $450\::\:\left[41\:-\:\left(2\:x\:-5\:\right)\right]=\:3^2\cdot 5$

$⇒-\dfrac{225}{x-23}=3^2\cdot \:5$

$⇒-\dfrac{225}{x-23}=9\cdot \:\:5$

$⇒-\dfrac{225}{x-23}=45$

$⇒-225=45\left(x-23\right)$

$⇒\dfrac{-225}{45}=x−23$

$⇒x-23=-5$

$⇒x=-5+23$

$⇒x=18$

$\text{Vậy x = 18}$

Answer

$\begin{array}{l} a, x - 65 = 38 - 58\\ \Rightarrow x - 65 = -20\\ \Rightarrow x = -20 + 65\\ \Rightarrow x = 45\\ \text{Vậy} \ x = 45\\ -------\\ b, 4x - 15 = -39\\ \Rightarrow 4x = -39 + 15\\ \Rightarrow 4x = -24\\ \Rightarrow x = -24 : 4\\ \Rightarrow x = -6\\ \text{Vậy} \ x = -6\\ -------\\ c, 450 : [41 - (2x - 5)] = 3^2 . 5\\ \Rightarrow 450 : [41 - (2x - 5)] = 9 . 5\\ \Rightarrow 450 : [41 - (2x - 5)] = 45\\ \Rightarrow [41 - (2x - 5)] = 450 : 45\\ \Rightarrow 41 - (2x - 5) = 10\\ \Rightarrow (2x - 5) = 41 - 10\\ \Rightarrow 2x - 5 = 31\\ \Rightarrow 2x = 31 + 5\\ \Rightarrow 2x = 36\\ \Rightarrow x = 36 : 2\\ \Rightarrow x = 18\\ \text{Vậy} \ x = 18\\ \end{array}$