5ˣ⁺¹ - 5ˣ = 100 . 25²⁹
5ˣ.(5 - 1) = 100 .(5²)²⁹
5ˣ.4 = 100.5⁵⁸
5ˣ = 100 . 5⁵⁸ : 4
5ˣ = 25 .5⁵⁸
5ˣ = 5².5⁵⁸
5ˣ = 5⁶⁰
x = 60
\(5^{x+1}-5^x=100\cdot25^{29}\\\Rightarrow 5^x\cdot5-5^x=2^2\cdot5^2\cdot(5^2)^{29}\\\Rightarrow 5^x\cdot(5-1)=4\cdot 5^2\cdot5^{58}\\\Rightarrow 4\cdot5^x=4\cdot 5^{60}\\\Rightarrow5^x=5^{60}\\\Rightarrow x=60\\Vậy:x=60\)