\(x:\left(\dfrac{1}{12}-\dfrac{3}{4}\right)=1\\ x:\left(-\dfrac{2}{3}\right)=1\\ x=-\dfrac{2}{3}\)

Số tiền mua 9 quyển vở là: 45000 : 5 x 9 = 81000 (đồng)

\(2x+1< 100\Rightarrow2x< 99\Rightarrow x< \dfrac{99}{2}=49,5\)

Mà \(x\in N\)* \(\Rightarrow x\in\left\{1;2;3;...;48;49\right\}\)