Tính số phần tử của tập hợp các số 2;5;8;11;...2015;2018;2021.
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Số hạng tổng quát của cấp số cộng là $u_n = u_1 + (n-1)d$.
Ta cần tìm $n$ sao cho $u_n = 2021$.
Vậy $2021 = 2 + (n-1)3$.
$2019 = (n-1)3$.
$n-1 = rac{2019}{3} = 673$.
$n = 673 + 1 = 674$.
Vậy số phần tử của tập hợp là 674.
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Sâu đục thân ngô:
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2. All devices' screens give out a large amount of blue light, which affects the body's production of the sleep hormone, melatonin, which can change your sleep patterns. So it is not just getting to sleep that becomes hard, but the quality of your sleep also suffers. The result is that you can wake up several times during the night, and have difficulty getting up in the morning.
3. Sleep is food for the brain, so not sleeping well can be a severe problem for anyone, especially for teenagers, because their brains are still developing. When you do not sleep well, it can be difficult to concentrate at school, which makes it harder to learn and get good grades. It can also affect your mood, making you feel sad or anxious.
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(Adapted from Prepare)
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