Tìm x biết 1,(x-7) =(x-7) 2,(5x-15) .(5x-15)=0 3,27<3x< 243 Bài 2 cho a =1+5+5+5+5+5 a 6 ; a 31

Đáp án:

$\begin{array}{l}
B1)\\
a)\left( {x - 7} \right) = \left( {x - 7} \right)\\
 \Leftrightarrow x - x =  - 7 + 7\\
 \Leftrightarrow 0 = 0\left( {tm} \right)\\
Vậy\,x \in Z\\
b)\left( {5x - 15} \right).\left( {5x - 15} \right) = 0\\
 \Leftrightarrow 5x - 15 = 0\\
 \Leftrightarrow 5x = 15\\
 \Leftrightarrow x = 3\\
Vậy\,x = 3\\
c)27 < 3x < 243\\
 \Leftrightarrow 9 < x < 81\\
Vậy\,9 < x < 81\\
B2)\\
A = 1 + 5 + {5^2} + {5^3} + {5^4} + {5^5}\\
 = \left( {1 + 5} \right) + \left( {{5^2} + {5^3}} \right) + \left( {{5^4} + {5^5}} \right)\\
 = 6 + {5^2}.6 + {5^4}.6\\
 = \left( {1 + {5^2} + {5^4}} \right).6 \vdots 6\\
 \Leftrightarrow A \vdots 6\\
A = 1 + 5 + {5^2} + {5^3} + {5^4} + {5^5}\\
 = \left( {1 + 5 + {5^2}} \right) + {5^3}.\left( {1 + 5 + {5^2}} \right)\\
 = 31 + {5^3}.31\\
 = \left( {1 + {5^3}} \right).31 \vdots 31\\
 \Leftrightarrow A \vdots 31
\end{array}$