Tìm x:

x/1.2 + x/2.3+ x/3.4 +...+ x/2017.2018 = -1

\(2.3^x-405=2.3.3^{x-1}-405=3^{x-1}\)

\(\Leftrightarrow6.3^{x-1}=3^{x-1}+405\Leftrightarrow5.3^{x-1}=405\Leftrightarrow3^{x-1}=81=3^4\)

\(\Leftrightarrow x-1=4\Leftrightarrow x=5\)

2.3^x - 405 = 3^x-1

2.3^x - 405 = 3^x : 3

2.3^x - 3^x : 3 = 405

\(\Leftrightarrow\)2 . 3^x - 3^x . \(\frac{1}{3}\)= 405

3^x . ( 2 - \(\frac{1}{3}\)) = 405

3^x . \(\frac{5}{3}\)= 405

3^x = 405 : \(\frac{5}{3}\)

3^x = 27

3^x = 3³

\(\Rightarrow\)x = 3

Bạn ghi đề lại câu a.

(2x-1)^{2 -} 81=0

(2x-1)² =0+81

(2x-1)² =81

(2x-1)² =9²

2x-1 =9

2x =9+1

2x =10

x =10:2

x =5