[tex]\displaystyle 5+9+13+...+101 \\ 101=5+(n-1) \times 4 \\ 101=5+4n-4 \\ 4n=101-5+4 \\ 4n=100 \\ n=100:4 \\ n=25 \\ S_{25}= \frac{10+24 \times 4}{2} \times 25 \\ \\ S_{25}= \frac{10+96}{2} \times 25 \\ \\ S_{25}= \frac{106}{2} \times 25 \\ \\ S_{25}=53 \times 25 \\ S_{25}=1325[/tex]
