giup minh 4/1x3+4/3x5+4/5x7+4/7x9+....+4/17x19+4/19x21
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Tính nhanh
2/1x3 + 2/3x5 + 2/5x7 + 2/7x9 +...+ 2/19x21
\(\frac{2}{1\times3}+\frac{2}{3\times5}+...+\frac{2}{19\times21}=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{19}-\frac{1}{21}\)
\(=1-\frac{1}{21}=\frac{20}{21}\)
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4/1x3 - 8/3x5 + 12/5x7 -16/7x9 + 20/9x11 - 24/11x13
giúp mình giải bài nay với ạ
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4/1x3 - 8/3x5 + 12/5x7 -16/7x9 + 20/9x11 - 24/11x13
giúp mình giải bài nay với ạ
A = \(\dfrac{4}{1\times3}\) - \(\dfrac{8}{3\times5}\) + \(\dfrac{12}{5\times7}\) - \(\dfrac{16}{7\times9}\) + \(\dfrac{20}{9\times11}\) - \(\dfrac{24}{11\times13}\)
A = ( \(\dfrac{1}{1}+\dfrac{1}{3}\)) - ( \(\dfrac{1}{3}\) + \(\dfrac{1}{5}\)) + (\(\dfrac{1}{5}\)+ \(\dfrac{1}{7}\)) - ( \(\dfrac{1}{7}\) + \(\dfrac{1}{9}\)) +( \(\dfrac{1}{9}\)+ \(\dfrac{1}{11}\)) - (\(\dfrac{1}{11}\)+\(\dfrac{1}{13}\))
A = \(\dfrac{1}{1}+\dfrac{1}{3}\) - \(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}+\dfrac{1}{7}\) - \(\dfrac{1}{7}\) - \(\dfrac{1}{9}\) + \(\dfrac{1}{9}\) + \(\dfrac{1}{11}\) - \(\dfrac{1}{11}\) - \(\dfrac{1}{13}\)
A = \(\dfrac{1}{1}\) - \(\dfrac{1}{13}\)
A = \(\dfrac{12}{13}\)
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Xem thêm câu trả lời
Tính thuận tiện:
a. 2345-2345:24:4 b. 1000-1000:8:125
123x25+123x74+123 2/1x3 + 2/3x5 + 2/5x7 + 2/7x9
a: =2345-2345/96=222775/96
b: =1000-1000:1000=1000-1=999
c: =123(25+74+1)=12300
d: =1-1/3+1/3-1/5+1/5-1/7+1/7-1/9=8/9
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a,1/1x3+1/3x5+1/5x7+1/7x9+1/X=1;b,2/3-1/3x(X-2)=1/4
a) \(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{x}=1\)
\(\Rightarrow\frac{1}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}\right)+\frac{1}{x}=1\)
\(\Rightarrow\frac{1}{2}.\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}\right)+\frac{1}{x}=1\)
\(\Rightarrow\frac{1}{2}.\left(1-\frac{1}{9}\right)+\frac{1}{x}=1\)
\(\Rightarrow\frac{1}{2}.\frac{8}{9}+\frac{1}{x}=1\)
\(\Rightarrow\frac{4}{9}+\frac{1}{x}=1\)
\(\Rightarrow\frac{1}{x}=1-\frac{4}{9}\)
\(\Rightarrow\frac{1}{x}=\frac{5}{9}\)
\(\Rightarrow x=\frac{1.9}{5}\)
\(\Rightarrow x=\frac{9}{5}\)
Vậy x = \(\frac{9}{5}\)
b) \(\frac{2}{3}-\frac{1}{3}.\left(x-2\right)=\frac{1}{4}\)
\(\Rightarrow\frac{1}{3}.\left(x-2\right)=\frac{2}{3}-\frac{1}{4}\)
\(\Rightarrow\frac{1}{3}.\left(x-2\right)=\frac{5}{12}\)
\(\Rightarrow x-2=\frac{5}{12}:\frac{1}{3}\)
\(\Rightarrow x-2=\frac{5}{4}\)
\(\Rightarrow x=\frac{5}{4}+2\)
\(\Rightarrow x=\frac{13}{4}\)
Vậy x = \(\frac{13}{4}\)
_Chúc bạn học tốt_
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tính bằng cách thuận tiện nhấtA= 1/1x3+1/3x5+1/5x7+1/7x9+......+1/19x21 Nhanh giùm mình nha! Cảm ơn trước nhé
tinh gia tri bieu thuc
S = 2/1x3-4/3x5+6/5x7-8/7x9+.....-96/95x97+98/97x99
tính bằng cách thuận tiện nhất A= 1/1x3+1/3x5+1/5x7+1/7x9+......+1/19x21 Nhanh giùm mình nha! Mình tick cho ai nhanh nhất nhé!
Lời giải:
$2\times A=\frac{2}{1\times 3}+\frac{2}{3\times 5}+\frac{2}{5\times 7}+...+\frac{2}{19\times 21}$
$2\times A=\frac{3-1}{1\times 3}+\frac{5-3}{3\times 5}+\frac{7-5}{5\times 7}+...+\frac{21-19}{19\times 21}$
$=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+....+\frac{1}{19}-\frac{1}{21}$
$=1-\frac{1}{21}=\frac{20}{21}$
$\Rightarrow A=\frac{20}{21}: 2= \frac{10}{21}$
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Tính tổng:
A=1/1+1/2+1/3+1/4+1/5+1/6
B=1x3+3x5+5x7+7x9+...+95x97+97x99
4/1x3+4/3x5+4/5x7+...+4/2009x2011