Question

1+3+5+...99=?
Urgent,va rog!

Answer 1

(1+99)×49:2
100×49:2
50×49
2450

Answer 2

$\displaystyle 1+3+5+...+99 \\ 99=1+(n-1) \times 2 \\ 99=1+2n-2 \\ 2n=99-1+2 \\ 2n=100 \\ n= 100:2 \\ n=50$

$\displaystyle S_{50}= \frac{2+49 \times 2}{2} \times 50 \\ \\ S_{50}= \frac{2+98}{2} \times 50 \\ \\ S_{50}= \frac{100}{2} \times 50 \\ \\ S_{50}=50 \times 50 \\ S_{50}=2500$