Tính: 1/2+1/2^3+1/2^5+1/2^7+...+1/2^99.
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10 tháng 8 2016
\(S=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{99.100}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+....+\frac{1}{99}-\frac{1}{100}\)
\(=1-\frac{1}{100}=\frac{99}{100}\)
DA
23 tháng 3 2020
1. 1 + ( -2) +3 +(-4) + .........+ 19 + (-20)
= -1 + ( -1) +....+(-1)
= -1. 10
= -10
2. 1 – 2 + 3 – 4 + . . . + 99 – 100
= ( -1) + (-1) +....+(-1)
= -1. 50
= -50
3. 2 – 4 + 6 – 8 + . . . + 48 – 50
= (-2) + (-2) +....+ (-2)
= -2. 12 + 26
= -24 + 26
= 2
4. – 1 + 3 – 5 + 7 - . . . . + 97 – 99
= 2 + 2 +......+2
= 2.25
= 50
5. 1 + 2 – 3 – 4 + ... + 97 + 98 – 99 - 100
= (1+2-3-4) +......+ ( 97+98-99 -100)
= -4 . (-4).....(-4)
= -4. 25
= -100
IN
1
6 tháng 12 2018
\(T=(\frac{1}{2}-\frac{1}{3})(\frac{1}{2}-\frac{1}{5})(\frac{1}{2}-\frac{1}{7}).....(\frac{1}{2}-\frac{1}{99})\)
\(\implies T=\frac{1}{2}(1-\frac{2}{3}).\frac{1}{2}(1-\frac{2}{5}).\frac{1}{2}(1-\frac{2}{7}).....\frac{1}{2}(1-\frac{2}{99})\)
Thấy T có: (99-3):2+1=49(SH)
\(\implies T=(\frac{1}{2}.49).[(1-\frac{2}{3}).(1-\frac{2}{5})...(1-\frac{2}{99})\)
\(\implies T=\frac{49}{2}.\frac{1}{99}=\frac{49}{198}\)
9 tháng 1 2022
\(S=1+\dfrac{1}{2}+\dfrac{1}{2^3}+\dfrac{1}{2^5}+...+\dfrac{1}{2^{101}}\)
\(\Rightarrow S-1=\dfrac{1}{2}+\dfrac{1}{2^3}+\dfrac{1}{2^5}+...+\dfrac{1}{2^{101}}\)
\(\Rightarrow\dfrac{1}{4}\left(S-1\right)=\dfrac{1}{2^3}+\dfrac{1}{2^5}+\dfrac{1}{2^7}+...+\dfrac{1}{2^{103}}\)
\(\Rightarrow\dfrac{1}{4}\left(S-1\right)-\left(S-1\right)=\dfrac{1}{2^3}+\dfrac{1}{2^5}+\dfrac{1}{2^7}+...+\dfrac{1}{2^{103}}-\dfrac{1}{2}-\dfrac{1}{2^3}-...-\dfrac{1}{2^{101}}\)
\(\Rightarrow\dfrac{3}{4}\left(S-1\right)=\dfrac{1}{2^{103}}\)
\(\Rightarrow S-1=\dfrac{1}{2^{103}}:\dfrac{3}{4}\)
\(\Rightarrow S-1=\dfrac{4}{3.2^{103}}\)
\(\Rightarrow S=\dfrac{4}{3.2^{103}}+1\)
DD
9 tháng 1 2022
S=1+12+123+125+...+12101S=1+12+123+125+...+12101
⇒S−1=12+123+125+...+12101⇒S−1=12+123+125+...+12101
⇒14(S−1)=123+125+127+...+12103⇒14(S−1)=123+125+127+...+12103
⇒14(S−1)−(S−1)=123+125+127+...+12103−12−123−...−12101⇒14(S−1)−(S−1)=123+125+127+...+12103−12−123−...−12101
⇒34(S−1)=12103⇒34(S−1)=12103
⇒S−1=12103:34⇒S−1=12103:34
⇒S−1=43.2103⇒S−1=43.2103
⇒S=43.2103+1
4 tháng 9 2020
Soái ca 2k6 Làm đi bạn !!
\(\frac{3^{2}+1}{3^{2}-1}+\frac{5^{2}+1}{5^{2}-1}+...+\frac{99^{2}+1}{99^{2}-1}=49+\frac{2}{2.4}+\frac{2}{4.6}+...+\frac{2}{98.100}=49+\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{98}-\frac{1}{100}=49.49\)
24 tháng 2 2018
a)\(1-2+3-4+5-6+7-8+8-9+9-10\)
=\(\left(1-2\right)+\left(3-4\right)+\left(5-6\right)+\left(7-8\right)+\left(8-9\right)+\left(9-10\right)\)
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)+\left(-1\right)\)
\(=\left(-1\right).6\)
\(=-6\)
b)\(1-2+3-4+...+99-100\)
\(=\left(1-2\right)+\left(3-4\right)+...+\left(99-100\right)\)}\(\left[\left(100-1\right):1+1\right]:2=50\)(cặp)
\(=\left(-1\right)+\left(-1\right)+\left(-1\right)+...+\left(-1\right)\)} 50 số (-1)
\(=\left(-1\right).50\)
\(=-50\)
c)\(1-3+5-7+9-11+13-15\)
\(=\left(1-3\right)+\left(5-7\right)+\left(9-11\right)+\left(13-15\right)\)
\(=\left(-2\right)+\left(-2\right)+\left(-2\right)+\left(-2\right)\)
\(=\left(-2\right).4\)
\(=-8\)
d)\(1-3+5-7+...-99+101\) (Đối với bài này, có vẻ đề sai, mình đã sửa lại rồi
\(=\left(1-3\right)+\left(5-7\right)+...+\left(97-99\right)+101\) } \(\left[\left(99-1\right):2+1\right]:2=25\)(cặp)
\(=\left(-2\right)+\left(-2\right)+\left(-2\right)+...+\left(-2\right)\) } 25 số (-2)
\(=\left(-2\right).25\)
\(=-50\)
e)\(-1-2-3-4-...-99-100\)
\(=\left(-1\right)+\left(-2\right)+\left(-3\right)+...+\left(-99\right)+\left(-100\right)\)
\(=\left[\left(-1\right)+\left(-100\right)\right]+\left[\left(-2\right)+\left(-99\right)\right]+...+\left[\left(-51\right)+\left(-50\right)\right]\) } \(\left[\left(100-1\right):1+1\right]:2=50\)(cặp) (phần này của đề bài, không thay được như (-100) hoặc (-1))
\(=\left(-100\right)+\left(-100\right)+\left(-100\right)+...+\left(-100\right)\)} 50 số (-100)
\(=\left(-100\right).50\)
\(=-5000\)
Đặt \(S=\frac{1}{2}+\frac{1}{2^3}+\frac{1}{2^5}+....+\frac{1}{2^{99}}\)
\(\Rightarrow\frac{1}{2^2}S=\frac{1}{2^3}+\frac{1}{2^5}+\frac{1}{2^7}+.....+\frac{1}{2^{101}}\)
\(\Rightarrow S-\frac{1}{4}S=\frac{1}{2}+\frac{1}{2^3}+\frac{1}{2^5}+....+\frac{1}{2^{99}}-\frac{1}{2^3}-\frac{1}{2^5}-\frac{1}{2^7}-....-\frac{1}{2^{101}}\)
\(\Rightarrow S\frac{1}{3}=\frac{1}{2}-\frac{1}{2^{101}}\)
\(\Rightarrow S=\frac{3}{2}-\frac{3}{2^{101}}\)
Vậy \(\frac{1}{2}+\frac{1}{2^3}+\frac{1}{2^5}+....+\frac{1}{2^{99}}=\frac{3}{2}-\frac{3}{2^{101}}\)