1. 25 . 3x-3 = 2025
3x-3 = 2025 : 25
3x-3 = 81
3x-3 = 34
=> x - 3 = 4
x = 4 + 3
x = 7
Vậy x = 7
2. Chứng minh:
M = 2 + 22 + 23 +...+298
M = ( 2 + 22 ) + ( 23 + 24 ) +...+ ( 297 + 298 )
M = 2.( 1 + 2 ) + 23.( 1 + 2 ) +...+ 297.( 1 + 2 )
M = 2.3 + 23.3 +...+ 297.3 \(⋮\)3
=> M\(⋮\)3
\(25.3^{x-3}=2025\)
\(3^{x-3}=2025:25\)
\(3^{x-3}=81\)
\(3^{x-3}=3^4\)
\(\Rightarrow x-3=4\)
\(\Rightarrow x=7\)
vay \(x=7\)