[tex]\large\boxed{\begin{array}{l}\rm(5,7,9\dotsc25)\\\rm a_n=a_1+(n-1)\cdot r\longrightarrow n=\dfrac{a_n-a_1}{r}+1\\\\\rm n=\dfrac{25-5}{2}+1\\\\\rm n=\dfrac{20}{2}+1\\\\\rm n=10+1\\\rm n=11\longrightarrow ter\acute a\,passado\,11\,anos\\\rm \underbrace{(125,125,125,\dotsc125)}_{\sf soma\,dos\,11\,elementos}\\\rm S_{11}=\dfrac{11\cdot(125+125)}{2}\\\\\rm S_{11}=\dfrac{11\cdot\diagdown\!\!\!\!\!\!\!\!250}{\backslash\!\!\!2}\\\rm S_{11}=11\cdot 125\\\rm S_{11}=1375\,figurinhas\end{array}}[/tex]
