Răspuns:
Explicație pas cu pas:
1/3*5 = 1/3 - 1/5
1/5*7 = 1/5 - 1/7
1/7*9 = 1/7 - 1/9
..........
1/2019*2021 = 1/2019 - 1/2021
_______________
1/3*5 + 1/5*7 + 1/7*9 + .....+ 1/2019*2021
= 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 +....+ 1/2019 - 1/2021
= 1/3 - 1/2021 = 2021/6063 - 3/6063 = 2018/6063
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