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\(\left(\frac{5}{10\cdot12}\right)+\left(\frac{5}{12\cdot14}\right)+...+\left(\frac{5}{146\cdot148}\right)\)
\(=5\left[\frac{1}{10\cdot12}+\frac{1}{12\cdot14}+...+\frac{1}{146\cdot148}\right]\)
\(=5\left[\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{4}+...+\frac{1}{146}-\frac{1}{148}\right]\)
\(=5\left[\frac{1}{10}-\frac{1}{148}\right]\)
\(=5\cdot\frac{69}{740}\)
\(=\frac{69}{148}\)
Không chắc nhá (:
@Trần Nhật Quỳnh@ lag
\(\frac{5}{10\cdot12}+\frac{5}{12\cdot14}+...+\frac{5}{146\cdot148}\)
\(=\frac{5}{2}\left(\frac{2}{10\cdot12}+\frac{2}{12\cdot14}+....+\frac{2}{146\cdot148}\right)\)
\(=\frac{5}{2}\left(\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}+...+\frac{1}{146}-\frac{1}{148}\right)\)
\(=\frac{5}{2}\left(\frac{1}{10}-\frac{1}{148}\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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2F = 2/10 x 12 + 2 / 12 x 14 + 2 / 14 x 16 + ... + 2 / 108 x 110
= 1/10 - 1/12 + 1/12 - 1/14 + 1/14 - 1/16 + ... + 1/108 -1/110
= 1/10 - 1/110
= 1/11
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\(E=\frac{2}{10x12}+\frac{2}{12x14}+...+\frac{2}{108x110}\)
\(\Rightarrow E=\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}+...+\frac{1}{108}-\frac{1}{110}\)
\(\Rightarrow E=\frac{1}{10}-\frac{1}{110}=\frac{10}{110}=\frac{1}{11}\)
Công thức :\(\frac{a}{n\left(n+a\right)}=\frac{1}{n}-\frac{1}{n+a}\)
\(E=\frac{2}{10\cdot12}+\frac{2}{12\cdot14}+\frac{2}{14\cdot16}+...+\frac{2}{108\cdot110}\)
\(E=\frac{1}{10\cdot12}+\frac{1}{12\cdot14}+\frac{1}{14\cdot16}+...+\frac{1}{108\cdot110}\)
\(E=\frac{1}{10}-\frac{1}{12}+\frac{1}{12}-\frac{1}{14}+\frac{1}{14}-\frac{1}{16}+...+\frac{1}{108}-\frac{1}{110}\)
\(E=\frac{1}{10}-\frac{1}{110}\)
\(E=\frac{11}{110}-\frac{1}{110}\)
\(E=\frac{10}{110}\)\(=\frac{1}{11}\)
Học tốt nha bạn !!!
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12 - 13 + 14 - 15 + 16
= -1+ (-1) + 16
= 0 + 16 = 16
18 - 16 + 14 - 12 + 10 - 8
= 2 + 2 + 2 = 6
12-13+14-15+16=(-1)+(-1)+16=(-2)+16=14
18-16+14-12+10-8=2+2+2=6
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\(\dfrac{18}{21}=\dfrac{6}{7}\)=\(\dfrac{12}{14}=\dfrac{36}{42}\)=\(\dfrac{90}{150}=\dfrac{3}{5}\)=\(\dfrac{12}{20}=\dfrac{60}{100}=\dfrac{21}{35}\)
10 + 12 + 14+... + 148 + 150
= 2( 5+6+7 +...+ 74 + 75)
= 2 [ (75 + 5)(75 - 5 +1) :2]
= 2 * 2760
= 5520
tổng là 5680