Question

calculati
a 3+6+9+12+...+2019
b 4+8+12+...+2020
c 5+10+15+20+...+2020
d 7+14+21+...+2023
rpdd dau coroana​

Answer 1

Explicație pas cu pas:

$a = \: 3(1 + 2 + 3 + 4 + .... + 673) \\ a = 3 \times \frac{673 \times 674}{2} \\ a = 3 \times \frac{673 \times 377}{1} \\ a = 644007 \\ b = 4( 1 + 2 + 3 + 4 + ..... + 505) \\ b = 4 \times \frac{505 \times 506}{2} \\ b = 4 \times 505 \times 253 \\ b = 511060 \\ c = 5(1 + 2 + 3 + 4 + ..... + 404) \\ c = 5 \times \frac{404 \times 405}{2} \\ c = 5 \times 202 \times 405 \\ c = 409050 \\ d = 7(1 + 2 + 3 + 4 + .... + 289) \\ d = 7 \times \frac{289 \times 290}{2} \\ d = 7 \times 289 \times 145 \\ d = 293335$