Question

1+3+5+...+19 calculati

Answer 1

$1+3+5+...+19 \\ \\ 19=1+(n-1) \times 2 \\ \\ 19=1+2n-2 \\ \\ 2n=19-1+2 \\ \\ 2n=20 \\ \\ n=20:2 \\ \\ n=10$

$\displaystyle S_1_0= \frac{2+9 \times 2}{2} \times 10 \\ \\ S_1_0= \frac{2+18}{2} \times 10 \\ \\ S_1_0= \frac{20}{2} \times 10 \\ \\ S_1_0=10 \times 10 \\ \\ S_1_0=100$