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

Đáp án:

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