\(25D=5^3+5^5+...+5^{203}\)
\(\Leftrightarrow24D=5^{203}-5\)
hay \(D=\dfrac{5^{203}-5}{24}\)
\(D=5+5^3+5^5+...+5^{201}\)
\(\Rightarrow5^2D=5^3+5^5+...+5^{203}\)
\(\Rightarrow25D-D=\left(5^3+5^5+...+5^{203}\right)-\left(5+5^3+...+5^{201}\right)\)
\(\Rightarrow24D=5^{203}-5\)
\(\Rightarrow D=\dfrac{5^{203}-5}{24}\)
`D = 5 + 5^3 + 5^5 + ... + 5^201`
`=> 5^2D = 5^3 + 5^5 + 5^7 + ... + 5^203`
`=> 25D - D = (5^3 - 5^5 + 5^7 + ... + 5^203) - (5 + 5^3 + 5^5 + 5^201)`
`=> 24D = 5^203 - 5`
`=> D = (5^203-5)/24`