Răspuns :
Suma=nr.termeni(ultimul termen+primul termen):2. Nr.termeni=(998-3+5):5=200. S=200(998+3):2=100100.
3+8+13+.....+998 =
= 5×1-2 + 5×2-2 + 5×3-2 +...+ 5×200 - 2 =
= 5×(1+2+3+...+200) - 2 × 200 =
= 5 × 200 × 201 : 2 - 400 =
= 5 × 100 × 201 - 400 =
= 100 500 - 400 =
= 100 100
= 5×1-2 + 5×2-2 + 5×3-2 +...+ 5×200 - 2 =
= 5×(1+2+3+...+200) - 2 × 200 =
= 5 × 200 × 201 : 2 - 400 =
= 5 × 100 × 201 - 400 =
= 100 500 - 400 =
= 100 100