Đáp án:
e) x=20
Giải thích các bước giải:
\(\begin{array}{l}
a)50 - \left[ {70:\left( {x - 1} \right)} \right] = 15\\
\to 70:\left( {x - 1} \right) = 35\\
\to x - 1 = 2\\
\to x = 3\\
b)({5^2} + {2^3}).x + {4^2} = {7^2}\\
\to \left( {25 + 8} \right)x + 16 = 49\\
\to 33x = 33\\
\to x = 1\\
c)2.x - 138 = {2^3}{.3^2}\\
\to 2x = 8.9 + 138\\
\to 2x = 210\\
\to x = 105\\
d)({2^6} - 4.x){.8^{39}} = {2.8^{40}}\\
\to 64 - 4x = 2.8\\
\to 4x = 48\\
\to x = 12\\
e)5.(x - 2) + 135 = {5^2}{.3^2}\\
\to 5\left( {x - 2} \right) = 25.9 - 135\\
\to 5\left( {x - 2} \right) = 90\\
\to x - 2 = 18\\
\to x = 20
\end{array}\)