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:

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