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NH
4
NQ
23 tháng 7 2015
A=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7
A=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
A=1-1/7
A=6/7
23 tháng 7 2015
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7
A = 1/1 - 1/2 + 1/2 - 1/3 + 1 /3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7
A = 1/1 - 1/7
A = 6/7
MH
23 tháng 1 2016
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
\(=1-\frac{1}{7}=\frac{6}{7}\)
23 tháng 1 2016
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}=\frac{45}{28}\)
29 tháng 8 2016
\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(A=1-\frac{1}{8}\)
\(A=\frac{7}{8}\)
29 tháng 8 2016
\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}\)
\(A=1-\frac{1}{8}\)
\(A=\frac{7}{8}\)
28 tháng 2 2015
Ta có
\(\frac{1}{2}=\frac{1}{1.2};\frac{1}{6}=\frac{1}{2.3};\frac{1}{12}=\frac{1}{3.4};\frac{1}{20}=\frac{1}{4.5};\frac{1}{30}=\frac{1}{5.6};\frac{1}{42}=\frac{1}{6.7}\)
\(\Rightarrow A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
Ta thấy:
\(\frac{1}{1.2}=1-\frac{1}{2};\frac{1}{2.3}=\frac{1}{2}-\frac{1}{3};...;\frac{1}{6.7}=\frac{1}{6}-\frac{1}{7}\)
\(\Rightarrow A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{6}-\frac{1}{7}\)
Thấy
\(-\frac{1}{2}+\frac{1}{2}=0;-\frac{1}{3}+\frac{1}{3}=0;...;-\frac{1}{6}+\frac{1}{6}=0\)
Ta coi như hết
\(\Rightarrow A=1-\frac{1}{7}\)
\(=\frac{6}{7}\)
NT
1
GB
2 tháng 10 2016
a, \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
= \(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
= \(1-\frac{1}{7}=\frac{6}{7}\)
NH
3
28 tháng 1 2016
=15/30+5/30+1/12+1/20+1/30+1/42
=2/3+1/12+1/20+1/30+1/42
=8/12+1/12+1/20+1/30+1/42
=3/4+1/20+1/30+1/42
=15/20+1/20+1/30+1/42
=4/5+1/30+1/42
=24/30+1/30+1/42
=5/6+1/42
=35/42+1/42
=6/7
28 tháng 1 2016
A=\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
A=\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}\)
A=\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
A=\(1-\frac{1}{7}\)
A=\(\frac{6}{7}\)
LN
16 tháng 8 2016
1 / 12 + 1 / 20 + 1 / 30 + 1 / 42 + 1 / 56 + 1 / 72 + 1 / 90 + 1 / 110 + 1 / 132 =
(2 310 * 1) / (2 310 * 12) + (1 386 * 1) / (1 386 * 20) + (924 * 1) / (924 * 30) + (660 * 1) / (660 * 42) + (495 * 1) / (495 * 56) + (385 * 1) / (385 * 72) + (308 * 1) / (308 * 90) + (252 * 1) / (252 * 110) + (210 * 1) / (210 * 132) =
2 310 / 27 720 + 1 386 / 27 720 + 924 / 27 720 + 660 / 27 720 + 495 / 27 720 + 385 / 27 720 + 308 / 27 720 + 252 / 27 720 + 210 / 27 720 =
( 2 310 + 1 386 + 924 + 660 + 495 + 385 + 308 + 252 + 210 ) / 27 720 = 6 930 / 27 720
Đề thiếu chắc mk làm máy bài này rồi !
NL
3
\(\Rightarrow A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
\(\Rightarrow A=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
\(\Rightarrow A=1-\frac{1}{7}=\frac{6}{7}\)
A=1/2 + 1/6 + 1/12 + 1/20 + 1/30 +1/42
A=1/1x2 + 1/2x3 +1/ 3x4 + 1/4x5 + 1/5x6 + 1/6x7
Nhận xét : 1/1x2=1/1-1/2 1/2x3=1/2-1/3 1/6x7=1/6-1/7
Ta có :
A= 1/1x2 +1/2x3 + 1/3x4 + 1/4/5 + 1/5x6 + 1/6x7
A= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6- 1/7
A=1/1 - 1/7
A=6/7
Vậy A=6/7
duyệt đi