Tìm x ,biết : a,9 mũ x-1=81 b,(19x+2.5 mũ 2):14=(13-8)mũ2 -4 mũ 2 c,2.3 mũ x =10.3 mũ 12 +8.27 mũ 4

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1. a, `9^(x-1) = 81`

   `9^(x-1) = 9^2`

   `x-1=2`

   `x=3`

   b, `(19x+2.5^2) : 14 = (13-8)^2 – 4^2`

   `(19x + 2.5^2) : 14 = 9`

   `19x + 2.5^2 = 126`

   `19x = 2.3^2.7 : 2.5^2`

   `19x = 2,52`

   `x=63/475`

   c. `2.3^x = 10.3^12 + 8.27^4`

   `2.3^x = 10.3^12 + 8.3^12`

   `2.3^x = 3^12 . (10+8)`

   `2.3^x = 18.3^12`

   `2.3^x = 2.3^2 .3^12`

   `2.3^x = 2.3^14`

   `x=14`

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2. Đáp án:

   $\begin{array}{l}
   a){9^{x – 1}} = 81\\
    \Rightarrow {9^{x – 1}} = {9^2}\\
    \Rightarrow x – 1 = 2\\
    \Rightarrow x = 2 + 1\\
    \Rightarrow x = 3\\
   \text{Vậy}\,x = 3\\
   b)\left( {19x + {{2.5}^2}} \right):14 = {\left( {13 – 8} \right)^2} – {4^2}\\
    \Rightarrow \left( {19x + 2.25} \right):14 = {5^2} – 16\\
    \Rightarrow \left( {19x + 50} \right):14 = 9\\
    \Rightarrow 19x + 50 = 9.14\\
    \Rightarrow 19x = 126 – 50\\
    \Rightarrow 19x = 76\\
    \Rightarrow x = 4\\
   \text{Vậy}\,x = 4\\
   c){2.3^x} = {10.3^{12}} + {8.27^4}\\
    \Rightarrow {2.3^x} = {10.3^{12}} + {8.3^{3.4}}\\
    \Rightarrow {2.3^x} = {10.3^{12}} + {8.3^{12}}\\
    \Rightarrow {2.3^x} = {3^{12}}.\left( {10 + 8} \right)\\
    \Rightarrow {2.3^x} = {3^{12}}.18\\
    \Rightarrow {2.3^x} = {3^{12}}{.3^2}.2\\
    \Rightarrow {2.3^x} = {2.3^{14}}\\
    \Rightarrow {3^x} = {3^{14}}\\
    \Rightarrow x = 14\\
   \text{Vậy}\,x = 14
   \end{array}$

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