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NT
Nguyễn Thị Thương Hoài
Giáo viên
VIP
13 tháng 1 2023
A= 1.2.3 + 2.3.4 + 3.4.5 +.....+ 98.99.100
4A = 98.99.100.4 + .....+ 3.4.5.4 + 2.3.4.4 + 1.2.3.4
4A = 98.99.100.(101-97) +... + 2.3.4.(5-1) + 1.2.3.4
4A = 98.99.100.101 - 97.98.99.100+......+2.3.4.5 - 1.2.3.4 + 1.2.3.4
4A = 98.99.100.101
A = 98.99.100.101 : 4
A = 24497550
A
7 tháng 9 2017
Ta có:
\(A=1.2.3+2.3.4+3.4.5+...+98.99.100\)
\(\Rightarrow4A=1.2.3.4+2.3.4.4+3.4.5.4+...+98.99.100.4\)
\(\Rightarrow4A=1.2.3.4+2.3.4.\left(5-1\right)+3.4.5.\left(6-2\right)+...+98.99.100.\left(101-97\right)\)
\(\Rightarrow4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...98.99.100.101-97.98.99.100\)
\(\Rightarrow4A=98.99.100.101\)
\(\Rightarrow A=\dfrac{98.99.100.101}{4}\)
Vậy \(A=\dfrac{98.99.100.101}{4}\)
C
7 tháng 9 2017
Ta có: \(A=1.2.3+2.3.4+3.4.5+...+98.99.100\)
\(4A=\left(1.2.3+2.3.4+...+98.99.100\right)4\)
\(4A=1.2.3.\left(4-0\right)+2.3.4.\left(5-1\right)...+98.99.100.\left(101-97\right)\)
\(4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...+98.99.100.101-97.98.99.100\)
\(4A=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+...+97.98.99.100-97.98.99.100+98.99.100.101\)
\(4A=98.99.100.101\)
\(\Rightarrow A=\dfrac{98.99.100.101}{4}=24497550\)
DN
5
NG
1
4 tháng 1 2019
Bạn cho sai đề rồi !
Sửa : Chứng tỏ : \(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{98.99.100}=\frac{4949}{9900}\)
Ta có : \(VT=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{98.99.100}\)
\(=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{98.99}-\frac{1}{99.100}\)
\(=\frac{1}{1.2}-\frac{1}{99.100}\)
\(=\frac{99.100-2}{2.99.100}\)
\(=\frac{4949}{9900}=VP\)
Study well ! >_<
DT
1
PT
4
14 tháng 4 2019
A=1/2 *(1/1*2-1/2*3+1/2*3-1/3*4+........+1/98*99-1/99*100)
=1/2*(1/2-1/99*100)
=1/2*(4950-1/9900)
=4950/19800
14 tháng 4 2019
\(A=\frac{1}{1\cdot2\cdot3}+\frac{1}{2\cdot3\cdot4}+\frac{1}{3\cdot4\cdot5}+...+\frac{1}{98\cdot99\cdot100}\)
\(A=\frac{1}{2}\left[\frac{2}{1\cdot2\cdot3}+\frac{2}{2\cdot3\cdot4}+\frac{2}{3\cdot4\cdot5}+...+\frac{2}{98\cdot99\cdot100}\right]\)
\(A=\frac{1}{2}\left[\frac{1}{1\cdot2}-\frac{1}{2\cdot3}+\frac{1}{2\cdot3}-\frac{1}{3\cdot4}+....+\frac{1}{98\cdot99}-\frac{1}{99\cdot100}\right]\)
\(A=\frac{1}{2}\left[\frac{1}{2}-\frac{1}{99\cdot100}\right]=\frac{1}{2}\cdot\frac{4949}{9900}=\frac{4949}{19800}\)
TT
11
22 tháng 3 2015
A=6+16+30+48+...+19600+19998
2A = 1.3+2.4+3.5+...+99.101
B=2+5+9+14+...+4949+5049
2A = 1.4+2.5+3.6+...+99.102
C=1.2.3+2.3.4+3.4.5+...+98.99.100
4A = 1.2.3.4+2.3.4(5-1)+3.4.5.(6-2)+...+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+...+98.99.100.101-97.98.99.100
4A = 98.99.100.101
C
21 tháng 9 2016
A=6+16+30+48+...+19600+19998
A : 2 = 3 + 8 + 15 + 24 + . . . + 9800 + 9999
A : 2 = 1.3 + 2.4 + 3.5 + 4.6 + . . . + 98.100 + 99.101
A : 2 = 1.[1+2] + 2.[1+3] + 3.[1+4] + 4.[1+5] + . . . + 98.[1+99] + 99.[1+100]
A : 2 = 1 + 1.2 + 2 + 2.3 + 3 + 3.4 + 4 + 4.5 + . . . + 98 + 98.99 + 99 + 99.100
A : 2 = 1 + 2 + 3 + 4 + . . . + 199 + 1.2 + 2.3 + 3.4 + 4.5 + . . . + 98.99 + 99.100
A : 2 = 4950 + 333300
A = 676500
NH
4
14 tháng 4 2019
A = 1.2.3 + 2.3.4 + 3.4.5 + ... + 98.99.100
4A = 1.2.3.(4-0) + 2.3.4(5-1) + 3.4.5.(6-2) + ... + 98.99.100.(101-97)
4A = 1.2.3.4 - 0.1.2.3 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + ... + 98.99.100.101 - 97.98.99.100
4A = (1.2.3.4 + 2.3.4.5 + 3.4.5.6 + ... + 98.99.100.101) - (0.1.2.3 + 1.2.3.4 + 2.3.4.5 + ... + 97.98.99.100)
4A= 98.99.100.101 - 0.1.2.3
4A = 98.99.100.101 - 0
4A = 98.99.100.101
A = 98.99.25.101
A = 24497550
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=\(\dfrac{\text{98.99.100.101}}{4}\)
tick nha
Ta có: \(A=1.2.3+2.3.4+3.4.5+...+98.99.100\)
\(4A=\left(1.2.3+2.3.4+...+98.99.100\right)4\)
\(4A=1.2.3.\left(4-0\right)+2.3.4.\left(5-1\right)...+98.99.100.\left(101-97\right)\)
\(4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...+98.99.100.101-97.98.99.100\)
\(4A=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+...+97.98.99.100-97.98.99.100+98.99.100.101\)
\(4A=98.99.100.101\)
\(\Rightarrow A=\dfrac{98.99.100.101}{4}=24497550\)