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\(A=\dfrac{1}{2}+\dfrac{2}{4}+\dfrac{3}{8}+...+\dfrac{10}{2^{10}}\)
\(2A=\dfrac{1}{1}+\dfrac{2}{2}+\dfrac{3}{4}+...+\dfrac{10}{2^9}\)
\(2A-A=\left(1+\dfrac{2}{2}+\dfrac{3}{4}+...+\dfrac{10}{2^9}\right)-\left(\dfrac{1}{2}+\dfrac{2}{4}+...+\dfrac{10}{2^{10}}\right)\)
\(A=1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2^9}-\dfrac{10}{2^{10}}\)
\(B=1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2^9}\)
\(2B=2+1+\dfrac{1}{2}+...+\dfrac{1}{2^8}\)
\(2B-B=\left(2+1+\dfrac{1}{2}+...+\dfrac{1}{2^8}\right)-\left(1+\dfrac{1}{2}+\dfrac{1}{4}+...+\dfrac{1}{2^9}\right)\)
\(B=2-\dfrac{1}{2^9}\)
Suy ra \(A=B-\dfrac{10}{2^{10}}=2-\dfrac{1}{2^9}-\dfrac{10}{2^{10}}=\dfrac{509}{256}\)
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a) \(\dfrac{-7}{5}\) . \(\dfrac{4}{23}\) + \(\dfrac{4}{23}\) . \(\dfrac{2}{5}\)
= \(\dfrac{4}{23}\) . ( \(\dfrac{-7}{5}\) + \(\dfrac{2}{5}\) )
= \(\dfrac{4}{23}\) . -1
= \(\dfrac{-4}{23}\)
b) \(\dfrac{-6}{7}\) . \(\dfrac{3}{5}\) +\(\dfrac{-6}{7}\) .\(\dfrac{2}{5}\) - \(1\dfrac{1}{3}\)
= \(\dfrac{-6}{7}\) . \(\dfrac{3}{5}\) +\(\dfrac{-6}{7}\) .\(\dfrac{2}{5}\) - \(\dfrac{4}{3}\)
= \(\dfrac{-6}{7}\) . (\(\dfrac{3}{5}\) + \(\dfrac{2}{5}\) ) - \(\dfrac{4}{3}\)
= \(\dfrac{-6}{7}\) . 1 - \(\dfrac{4}{3}\)
= \(\dfrac{-6}{7}\) - \(\dfrac{4}{3}\)
= \(\dfrac{-18}{21}\) - \(\dfrac{28}{21}\)
= \(\dfrac{-18}{21}\) + \(\dfrac{-28}{21}\)
= \(\dfrac{-46}{21}\)
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- Tất cả các đoạn thẳng có trong hình vẽ là : MP ; MQ ; MN ; NQ ; PQ ; NP
\(\Rightarrow\) Chọn \(B\)
_HT_
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1, 11. (2+4+6+8) + 89.(2+4+6+8)
= (11+89). (2+4+6+8)
= 100. 20
= 2000
2, 18. (1+3+5+7) + 82. (1+3+5+7)
= (18+82). (1+3+5+7)
= 100. 16
= 1600
3, 54. (5+10+15) + 46. (15+10+5)
= (54+46). (5+10+15)
= 100. 30
= 3000
4, 39. (4+7+9) + (9+7+4). 61
= (4+7+9). (39+61)
= 20. 100
= 2000
Lời giải:
1.
$11(2+4+6+8)+89(2+4+6+8)=(2+4+6+8)(11+89)$
$=20.100=2000$
2.
$18(1+3+5+7)+82(1+3+5+7)=(1+3+5+7)(18+82)$
$=16.100=1600$
3.
$54(5+10+15)+46(15+10+5)=(5+10+15)(54+46)$
$=30.100=3000$
4.
$39(4+7+9)+(9+7+4).61=(4+7+9)(39+61)$
$=20.100=2000$