3. [tex]<=>log_{2}^{2}x-8\sqrt{log_2x+3}-12=0[/tex]
đặt [tex]\sqrt{log_2x+3}=t=>t^4=log_{2}^{2}x+6.log_2x+9=>t^4-6t^2-8t-2=0[/tex]
5.[tex]<=>log_3({2x+1})=\frac{2}{log_3({2x+1})}+1<=>log_3^2({2x+1})-log_3({2x+1})-2=0[/tex]
đặt [tex]log_3({2x+1})=t=>t^2-t-2=0[/tex]