Question

Efectuati 4+9+14+19+...+1004.
VA ROG EU MULT CAT SE POATE DE REPEDE

Answer 1

$\displaystyle 4+9+14+19+...+1004 \\ 1004=4+(n-1) \times 5 \\ 1004=4+5n-5 \\ 5n=1004-4+5 \\ 5n=1005 \\ n=1005:5 \\ n=201 \\ S_{201}= \frac{8+200 \times 5}{2} \times 201 \\ \\ S_{201}= \frac{8+1000}{2} \times 201 \\ \\ S_{201}= \frac{1008}{2} \times 201 \\ \\ S_{201}=504 \times 201 \\ S_{201}=101304$