Răspuns:
Explicație pas cu pas:
a)
3 + 6 + 9 + 12 + ...... + 2019 =
= 3 × ( 1 + 2 + 3 + 4 + ...... + 673) =
= 3 × 673 × ( 1 + 673 ) : 2 =
= 2019 × 674 : 2 =
= 2019 × 337 =
= 680 403
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b)
4 + 8 + 12 + ..... + 2020 =
= 4 × ( 1 + 2 + 3 + ..... + 505 ) =
= 4 × 505 × ( 1 + 505 ) : 2 =
= 2 × 505 × 506 =
= 511 060
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c)
5 + 10 + 15 + ..... + 2020 =
= 5 × ( 1 + 2 + 3 + ...... + 404 ) =
= 5 × 404 × ( 1 + 404 ) : 2 =
= 5 × 202 × 405 =
= 409 050
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d)
7 + 14 + 21 + ..... 2023 =
= 7 × ( 1 + 2 + 3 + ..... + 289 ) =
= 7 × 289 × ( 1 + 289 ) : 2 =
= 2023 × 290 : 2 =
= 2023 × 145 =
= 293 335