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H
2
BT
23 tháng 11 2017
A=1.2+2.3+3.4+4.5+...+2017.2018
=> 3A=1.2.3+2.3.3+3.4.3+4.5.3+...+2017.2018.3
3A=1.2.3+2.3(4-1)+3.4(5-2)+4.5(6-3)+...+2017.2018.(2019-2016)
3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+2017.2018.2019-2016.2017.2018
3A=(1.2.3+2.3.4+3.4.5+4.5.6+...+2017.2018.2019)-(1.2.3+2.3.4+3.4.5+...+2016.2017.2018)
=> 3A=2017.2018.2019 => \(A=\frac{2017.2018.2019}{3}\); \(B=\frac{2018^3}{3}=\frac{2018.2018.2018}{3}\)
Ta có: 2017.2019=2017(2018-1)=2017.2018+2017<2017.2018+2018=2018(2017+1)=2018.2018
=> 2017.2018.2019<2018.2018.2018
=> A<B
16 tháng 11 2018
Bui The Hao lam dung roi
mk cung dang can bai nay
Thanks vi da dang honganh
DN
1
PT
23 tháng 11 2017
\(A=1.2+2.3+3.4+...+2017.2018\)
\(3A=1.2.3+2.3.3+3.4.3+...+2017.2018.3\)
\(3A=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+2017.2018.\left(2019-2016\right)\)
\(3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+2017.2018.2019-2016.2017.2018\)
\(\Rightarrow3A=2017.2018.2019\)
\(\Rightarrow A=\dfrac{2017.2018.2019}{3}\)
\(A=\dfrac{2017.2018.2019}{3};B=\dfrac{2018^3}{3}=\dfrac{2018.2018.2018}{3}\)
\(A=2739315938;B=2739316611\)
\(\Rightarrow A< B\)
ST
1
26 tháng 11 2017
Ta có : A=1.2+2.3+3.4+....+2015.2016
=>3A= 1.2.3 + 2.3.3 + 3.4.3 + 4.5.3 + ... + 2017.2018.3
=>3A= 1.2.3 + 2.3.( 4 - 1 ) + 3.4.( 5-2 ) + 4.5.( 6-3 ) + ... 2017 . 2018 . ( 2019 - 2016 )
=>3A=-1.2.3 + 2.3.4 - 2.3.1 + 3.4.5 - 3.4.2 + 4.5.6 - 4.5.3 +.....+ 2017 . 2018 .2019 - 2017 . 2018 . 2016
=>A= 2017 . 2018 . 2019
NN
0
HN
1
8 tháng 7 2019
A = 1.2 + 2.3 + 3.4 + ....... + 99.100
3A = 1.2.3 + 2.3.3 + 3.4.3 + ....... + 99 . 100 . 3
3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) +.... + 99.100.(101-98)
3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ..... + 99 . 100 . 101 - 98 . 99 . 100
3A = (1.2.3 - 1.2.3) + (2.3.4-2.3.4) + ... + (98.99.100 - 98.99.100) + 99 . 100 . 101
3A = 99 . 100 . 101 = 999900
A = 999900 : 3
A = 333300
KG
Cho A=1/1.2 + 1/2.3 + + 1/ 3.4+...+1/49.50 ; B = 1.2+2.3+3.4+4.5+5.6+...+49.50
Tính 50 mủ 2 A – B/17
0
31 tháng 7 2023
a) \(1.2+2.3+3.4+...+19.20\)
\(=\dfrac{20.\left(20+1\right).\left(20+2\right)}{3}\)
\(=3080\)
b) \(9+99+999+...+999...9\left(100so9\right)\)
\(\)\(=\left(10-1\right)+\left(100-1\right)+\left(1000-1\right)+...+\left(1000...0-1\right)\left(99so0\right)\)
\(=\left(10+10^2+10^3+...10^{99}\right)+\left(-1\right).100\)
\(=\left(1+10+10^2+10^3+...10^{99}\right)+\left(-1\right).101\)
\(=\dfrac{10^{99+1}-1}{99-1}-101\)
\(=\dfrac{10^{100}-1}{98}-101\)
\(=\dfrac{10^{100}-9899}{98}\)
31 tháng 7 2023
c) \(999.9x222...2\) (100 số 9; 100 số 2)
\(9x2=18\)
\(99x22=2178\)
\(999x222=\text{221778}\)
\(9999x2222=22217778\)
\(99999x22222=2222177778\)
\(.........\)
Theo quy luật trên ta có 100 số 9 nhân 100 số 2:
\(999.9x222...2=222...21777...78\) (99 sô 2; 1 số 1; 99 số 7; 1 số 8)
cho bài kham khảo nè :
A=1.2+2.3+3.4+4.5+...+2017.2018
=> 3A=1.2.3+2.3.3+3.4.3+4.5.3+...+2017.2018.3
3A=1.2.3+2.3(4-1)+3.4(5-2)+4.5(6-3)+...+2017.2018.(2019-2016)
3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+2017.2018.2019-2016.2017.2018
3A=(1.2.3+2.3.4+3.4.5+4.5.6+...+2017.2018.2019)-(1.2.3+2.3.4+3.4.5+...+2016.2017.2018)
=> 3A=2017.2018.2019 => \(A=\frac{2017.2018.2019}{3};B=\frac{2018^3}{3}=\frac{2018.2018.2018}{3}\)
Ta có: 2017.2019=2017(2018-1)=2017.2018+2017<2017.2018+2018=2018(2017+1)=2018.2018
=> 2017.2018.2019<2018.2018.2018
=> A<B
thank nha
A=1.2+2.3+3.4+...+2017.2018
3A=1.2.3+2.3.3+3.4.3+...+2017.2018.3
3A=1.2.3+2.3.(4−1)+3.4.(5−2)+...+2017.2018.(2019−2016)
3A=1.2.3+2.3.4−1.2.3+3.4.5−2.3.4+...+2017.2018.2019−2016.2017.2018
⇒3A=2017.2018.2019
⇒A=2017.2018.20193
A=2017.2018.20193;B=201833=2018.2018.20183
A=2739315938;B=2739316611
⇒A<B