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MỚI LÀM LÚC TỐI,HÊN QUÁ:
\(A=\frac{1}{3}+\frac{2}{3^2}+\frac{3}{3^3}+...+\frac{100}{3^{100}}\)
\(3A=1+\frac{2}{3}+\frac{3}{3^2}+...+\frac{100}{3^{99}}\)
\(2A=1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^{99}}-\frac{100}{3^{100}}\)
\(6A=3+1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\)
\(4A=3-\left(\frac{101}{3^{99}}-\frac{100}{3^{100}}\right)\)
\(4A=3-\frac{203}{3^{100}}\)
\(A=\frac{3}{4}-\frac{203}{3^{100}\cdot4}< \frac{3}{4}\)
Ta rút gọn 2 ở dưới vs 2 ở trên, rồi 3 ở dưới vs 3 ở trên cứ tiếp tục như vậy thì còn số 1/100, đó là kp của mình.
đặt \(A=\frac{1}{3}-\frac{2}{3^2}+\frac{3}{3^3}-\frac{4}{3^4}+...+\frac{99}{3^{99}}-\frac{100}{3^{100}}\)
\(\Rightarrow3A=1-\frac{2}{3}+\frac{3}{3^2}-\frac{4}{3^3}+...+\frac{99}{3^{98}}-\frac{100}{3^{99}}\)
\(\Rightarrow A+3A=\left(\frac{1}{3}-\frac{2}{3^2}+\frac{3}{3^3}-\frac{4}{3^4}+...+\frac{99}{3^{99}}-\frac{100}{3^{100}}\right)+\left(1-\frac{2}{3}+\frac{3}{3^2}-\frac{4}{3^3}+...+\frac{99}{3^{98}}-\frac{100}{3^{99}}\right)\)
\(\Rightarrow4A=1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}-\frac{100}{3^{100}}\)<\(B=1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\)
\(\Rightarrow3B=3-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{97}}-\frac{1}{3^{98}}\)
\(\Rightarrow B+3B=\left(1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\right)+\left(3-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{97}}-\frac{1}{3^{98}}\right)\)
\(\Rightarrow4B=3-\frac{1}{3^{98}}
(2/3×x-1/3)=2/3+1/3
(2/3×x-1/3)=3/3
2/3×x=3/3+1/3
2/3×x=4/3
x=4/3:3/2
x=4/3×2/3
x=8/9
a)\(\frac{32}{64}-\frac{16}{64}+\frac{8}{64}-\frac{4}{64}+\frac{2}{64}-\frac{1}{64}\le\frac{1}{3}\)
\(\Rightarrow\frac{32-16+8-4+2-1}{64}=\frac{23}{64}\)\
\(\Rightarrow\frac{23}{64}=0,359375;\frac{1}{3}=0,33333...\)
đề sao lạ vậy
\(a)\frac{-1}{8}+\frac{-5}{3}\) \(b)\frac{-6}{35}.\frac{-49}{54}\)
\(=\frac{-3}{24}+\frac{-40}{24}\) \(=\frac{\left(-6\right).\left(-49\right)}{35.54}\)
\(=\frac{-43}{24}\) \(=\frac{7}{45}\)
\(c)\frac{-4}{5}:\frac{3}{4}\)
\(=\frac{-4}{5}.\frac{4}{3}\)
\(=\frac{-16}{15}\)
#)Giải :
\(A=\frac{1}{3^1}-\frac{1}{3^2}+\frac{1}{3^3}-\frac{1}{3^4}+...+\frac{1}{3^{99}}-\frac{1}{3^{100}}\)
\(A=\frac{2}{9^1}+\frac{2}{9^2}+\frac{2}{9^3}+...+\frac{2}{9^{50}}\)
\(\Rightarrow2A=1+\frac{2}{9}+\frac{2}{9^1}+\frac{2}{9^2}+\frac{2}{9^3}+...+\frac{2}{9^{49}}\)
\(\Rightarrow2A-A=A=\left(1+\frac{2}{9}+\frac{2}{9^1}+\frac{2}{9^2}+\frac{2}{9^3}+...+\frac{2}{9^{49}}\right)-\left(\frac{2}{9^1}+\frac{2}{9^2}+\frac{2}{9^3}+...+\frac{2}{9^{50}}\right)\)
\(\Rightarrow A=1+\frac{2}{9}-\frac{2}{9^{50}}=\frac{11}{9}-\frac{2}{9^{50}}\)
Có lẽ đúng .........................
Ta có :
\(A=\frac{1}{3}-\frac{2}{3^2}+\frac{3}{3^3}-\frac{4}{3^4}+...+\frac{99}{3^{99}}-\frac{100}{3^{100}}\)
\(\Rightarrow3A=1-\frac{2}{3}+\frac{3}{3^2}-\frac{4}{3^3}+...+\frac{99}{3^{98}}-\frac{100}{3^{99}}\)
\(\Rightarrow4A=1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}-\frac{100}{3^{100}}\)
Đặt 4A = C
\(\Rightarrow3C=3-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{97}}-\frac{1}{3^{98}}-\frac{100}{3^{99}}\)
\(\Rightarrow4C=3-\frac{1}{3^{99}}-\frac{100}{3^{100}}-\frac{100}{3^{99}}\)
\(\Rightarrow4C< 3\Rightarrow C< \frac{3}{4}\Rightarrow4A< \frac{3}{4}\Rightarrow A< \frac{3}{16}\left(đpcm\right)\)
Ta có :
1/2 + 2/3 + 3/4 + .... + 99/100
= (2/2 − 1/2) + (3/3 − 1/3) + (4/4 − 1/4) + .... + (100/100 − 1/100)
= 1 − 1/2 + 1/2 − 1/3 + 1/3 − 1/4 + .... + 1/99 − 1/100
= 1 − 1/100
Vì : 1/100 > 0 ⇒ 1 − 1/100 < 1
Vậy 1/2 + 2/3 + 3/4 + ... + 99/100 < 1 (đpcm)
B=12!12!+23!+34!23!+34!+...+99100!99100!
=2−12!2−12!+3−13!+4−14!3−13!+4−14!+...+100−1100!100−1100!
=22!−12!+33!−13!+44!−14!+...+100100!−1100!22!−12!+33!−13!+44!−14!+...+100100!−1100!
=11!−12!+12!−13!+13!−14!+...+199!−1100!11!−12!+12!−13!+13!−14!+...+199!−1100!
=1−1100!1−1100!< 1
⇒⇒B =12!12!+23!+34!23!+34!+...+99100!99100! < 1