Đặt A=1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100

4A=(1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100)4

4A=1.2.3(4-0)+2.3.4(5-1)+3.4.5(6-2)+4.5.6(7-3)+...+98.99.100(101-97)

4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+4.5.6.7-3.4.5.6+...+98.99.100.101-97.98.99.100

4A=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+3.4.5.6-3.4.5.6+...+97.98.99.100-97.98.99.100+98.99.100.101

4A=98.99.100.101

=>A=98.99.100.101/4

TICK ĐÚNG GIÚP MÌNH Ặ

\(2S=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{99.100}\)

\(2S=\frac{1}{2}-\frac{1}{9900}\)

\(2S=\frac{4949}{9900}\)

\(S=\frac{4949}{19800}\)

Ta xét : \(\frac{1}{1.2}-\frac{1}{2.3}=\frac{2}{1.2.3}\)

\(\frac{1}{2.3}-\frac{1}{3.4}=\frac{2}{2.3.4}\)

...

\(\frac{1}{98.99}-\frac{1}{99.100}=\frac{2}{98.99.100}\)

Ta có : 2S = \(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{98.99}-\frac{1}{99.100}\)

=> 2S = \(\frac{1}{1.2}-\frac{1}{99.100}\)

=> 2S = \(\frac{4949}{9900}\)

=> S = \(\frac{4949}{19800}\)

2S=\(\dfrac{2}{1.2.3}+\dfrac{2}{2.3.4}+...+\dfrac{2}{98.99.100}\)

2S= \(1-\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{2}-\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{99}-\dfrac{1}{100}\)2S= 1- \(\dfrac{1}{100}\)

2S= \(\dfrac{99}{100}\)

S= \(\dfrac{99}{100}.\dfrac{1}{2}\)

S=\(\dfrac{198}{100}\)

Đặt A=1.2.3+2.3.4+.....+5.6.7

4A=1.2.3.4+2.3.4.(5-1)+.........+5.6.7.(8-4)

4A=1.2.3.4+2.3.4.5-1.2.3.4+........+5.6.7.8-4.5.6.7

4A=5.6.7.8

A=5.6.7.8:4

A=420

A = 1.2.3 + 2.3.4 + 3.4.5 + ... + 20.21.22

4A = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + ... + 20.21.22.4

4A = 1.2.3.4 + 2.3.4.(5 - 1) + 3.4.5.(6 - 2) + ... + 20.21.22.(23 - 19)

4A = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + ... + 20.21.22.23 - 19.20.21.22

4A = 20.21.22.23

A = 20.21.22.23 : 4

A = 53130

Đặt A = 1.2.3 + 2.3.4 + 3.4.5 + 4.5.6 +...+ 20.21.22

\(\Rightarrow4A=1.2.3.4+2.3.4.4+3.4.5.4+4.5.6.4+...+20.21.22.4\)

       \(=1.2.3.4+2.3.4.\left(5-1\right)+3.4.5.\left(6-2\right)+4.5.6.\left(7-3\right)+...+20.21.22.\left(23-19\right)\)  

        = 1.2.3.4 + 2.3.4.5  -1.2.3.4 + 3.4.5.6 - 2.3.4.5 + 4.5.6.7 - 3.4.5.6 +...+ 20.21.22.23 - 19.20.21.22

        =    20.21.22.23

        =  212520

_Hok tốt_

!!!        

đặt A=1.2.3+2.3.4+3.4.5+...+98.99.100

A.4=1.2.3.4+2.3.4.4+3.4.5.4+..+98.99.100.4

A.4=1.2.3(4-0)+2.3.4.(5-1)+3.4.5.(6-2)+......+98.99.100.(101-97)

a.4=1.2.3.4-0+2.3.4.5-2.3.4+3.4.5.6-2.3.4.5+...+98.99.100.101-97.98.99.100

A.4=98.99.100.101

A.4=97990200

A= 97990200:4

A=            24497550

Phải là phân số \(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{98.99.100}\) chứ !

Đặt S=1.2.3+2.3.4+...+98.99.100

=>4S=1.2.3.4+2.3.4.4+...+98.99.100.4

=>3S=1.2.3(4-0)+2.3.4(5-1)+....+98.99.100(101-97)

=>4S=1.2.3.4-0.1.2.3+2.3.4.5-1.2.3.4+....+98.99.100.101-97.98.99.100

=>4S=98.99.100.101

=>S=24497550

đặt S=1.2.3+2.3.4+....+47.48.49

4S=1.2.3.(4-0)+2.3.4.(5-1)+...+47.48.49.(50-46)

4S=1.2.3.4-1.2.3+2.3.4.5-1.2.3.4+....+47.48.49.50-46.47.48.49

4S=47.48.49.50-1.2.3

S=(47.48.49.50-1.2.3):4

cool queen đúng rồi

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