1phan2 + 1phan6 + 1phan12 +1phan20 + 1phan30 +1phan42 +1phan42 +1phan56 +1phan72 =?//???[cach lam luon nhe]
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\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{132}\)
\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{11.12}\)
\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{11}-\frac{1}{12}\)
\(=\frac{1}{1}-\frac{1}{12}\)
\(=\frac{11}{12}\)
\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{132}\)
\(A=\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+...+\frac{1}{11x12}\)
\(A=\frac{2-1}{1x2}+\frac{3-2}{2x3}+\frac{4-3}{3x4}+\frac{5-4}{4x5}+...+\frac{12-11}{11x12}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{11}-\frac{1}{12}\)
\(A=1-\frac{1}{12}=\frac{11}{12}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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27,135 + \(\frac{71}{100}+\frac{542}{1000}\)
= 27,135 + 0,71 + 0,542
= 27,845 + 0,542
= 28,387
a, = 27135/1000 + 710/1000 + 542/1000 = (27135+710+542)/1000 = 28387/1000 = 28,387
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\(C1:\)
\((6,7 + 2,3 + 5,8) : 1,2\)
\(= 14,8 : 1,2\)
\(= \frac {37}{3}\)
\(C2:\)
\((6,7+2,3+5,8) : 1,2 \)
\(= 6,7:1,2 + 2,3:1,2+5,8:1,2\)
\(= \frac {67}{12} + \frac {23}{12} + \frac {29}{6}\)
\(= \frac {37}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Giải
số a là :
( 5 + 2 ) : 2 = 3,5
số x là :
5 - 3,5 = 1,5
đáp số : a = 3,5
x = 1,5
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)
=\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)
=\(\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+\frac{5-4}{4.5}+\frac{6-5}{5.6}+\frac{7-6}{6.7}+\frac{8-7}{7.8}+\frac{9-8}{8.9}\)
=\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)
=\(1-\frac{1}{9}=\frac{8}{9}\)
1/2 + 1/6 + 1/12 + ... + 1/72
= 1/1.2 + 1/2.3 + 1/3.4 + ... + 1/8.9
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/8 - 1/9
= 1 - 1/9
= 8/9