tinh A = 1.2 + 2.3+3.4+....199:200
A=\(\frac{1.2+2.3+3.4+.....+20.21}{1+2-3-4+5+6-7-8+......+197+198-199-200+201}\)
Tính A
\(A=\frac{1\cdot2+2\cdot3+3\cdot4+...+20\cdot21}{1+2-3-4+5+6-7-8+...+197+198-199-200+201}\) (1)
đặt \(B=1\cdot2+2\cdot3+3\cdot4+...+20\cdot21\)
\(3B=1\cdot2\cdot3+2\cdot3\cdot3+3\cdot4\cdot3+...+20\cdot21\cdot3\)
\(3B=1\cdot2\cdot\left(3-0\right)+2\cdot3\cdot\left(4-1\right)+3\cdot4\cdot\left(5-2\right)+...+20\cdot21\cdot\left(22-19\right)\)
\(3B=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+3\cdot4\cdot5-2\cdot3\cdot4+...+20\cdot21\cdot22-19\cdot20\cdot21\)
\(3B=20\cdot21\cdot22\)
\(B=\frac{20\cdot21\cdot22}{3}=3080\) (2)
đặt \(C=1+2-3-4+5+6-7-8+...+197+197-199-200+201\)
\(C=\left(1+2-3-4\right)+\left(5+6-7-8\right)+...+\left(197+198-199-200\right)+201\)
\(C=-4+\left(-4\right)+...+\left(-4\right)+201\) có 50 số -4
\(C=-4\cdot50+201\)
\(C=-200+201\)
\(C=1\) (3)
\(\left(1\right)\left(2\right)\left(3\right)\Rightarrow A=\frac{B}{C}=\frac{30801}{1}=3080\)
Tính :
\(\dfrac{1.2+2.3+3.4+...+20.21}{1+2-3-4+5+6-7-8+...+197+198-199-200+201}\)
tinh tong A=1.2+2.3+3.4+4.5+...+2014.2015
A=1.2+2.3+3.4+4.5+...+2014.2015
=>3A=1.2.3+2.3.3+3.4.3+4.5.3+...+2014.2015.3
=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+2014.2015.(2016-2013)
=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+2014.2015.2016-2013.2014.2015
=(1.2.3-1.2.3)+(2.3.4-2.3.4)+(3.4.5-3.4.5)+(4.5.6-4.5.6)+...+(2013.2014.2015-2013.2014.2015)+0.1.2+2014.2015.2016
=0+2014.2015.2016
=>A=\(\frac{2014.2015.2016}{3}\)
TINH
A=1.2+2.3+ 3.4+...+99.100
3A = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2) + ... + 99.100.(101-98)
3A = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 99.100.101 - 98.99.100
3A = 99.100.101 - 0.1.2
3A = 99.100.101
A = 33.100.101
A = 333300
Tinh A = 1.2+2.3+3.4+...+n.(n+1)
Bài giải:
3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + n(n +1)3
= 1.2.(3 - 0) + 2.3.(4 - 1) + 3.4.(5 - 2) + ...+ n(n + 1)[(n + 2) - (n -1)]
= 1.2.3 + 2.3.4 - 2.3 + 3.4.5 - 2.3.4 + ... + n(n + 1)(n + 2) - n(n + 1)(n - 1)
= n(n + 1)(n + 2)
=> S N(N+1)(n+2)/3
3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + n(n +1)3
= 1.2.(3 - 0) + 2.3.(4 - 1) + 3.4.(5 - 2) + ...+ n(n + 1)[(n + 2) - (n -1)]
= 1.2.3 + 2.3.4 - 2.3 + 3.4.5 - 2.3.4 + ... + n(n + 1)(n + 2) - n(n + 1)(n - 1)
= n(n + 1)(n + 2)
=> S = \(\frac{n\left(n+1\right)\left(n+2\right)}{3}\)
tinh tong A=1.2+2.3+3.4+4.5+5.6+...+2016.2017
A=1.2+2.3+3.4+4.5+5.6+...+2016.2017
=> 3A = 1.2.3+2.3.3+3.4.3+4.5.3+5.6.3+.......+2016.2017.3
=> 3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + 4.5.(6-3) + .......+ 2016.2017.(2018-2015)
=> 3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +..........+ 2016.2017.2018 - 2015.2016.2017
=> 3A = 2016.2017.2018
=> A = 2016.2017.2018 : 3
Ta thấy:Các số trong dãy số trên cách nhau 1,1 đơn vị.
Số các số hạng là:
( 2016,2017 - 1,2 ) : 1,1 + 1 = 1832,819727 ( số )
Tổng là:
( 2016,2017 + 1,2 ) x 1832,819727 : 2 = 1848766,817
Đ/S: số trên dài wóa :))
tinh
A=1.2+2.3+3.4+....+N(N+1)
3A=1.2.3+2.3.3+3.4.3+...+n(n+1).3
3A=1.2(3-0)+2.3(4-1)+3.4(5-3)+....+n(n+1)(n-2)-(n-1)
3A=1.2.3-1.2.0+2.3.4-2.3.3+.+n(n+1)+(n+2)-(n-1)+n(n-1)
=>n(n-1)+(n+2)=\(\frac{n\left(n-1+\left(n+2\right)\right)}{3}\)
3A=1.2.3+2.3.3+3.4.3+.....+N(N+1).3
3A=1.2(3-0)+2.3(4-1)+3.4(5-3)+........+n(n+1)(n-2)-(n-1)
3a=1.2.3-1.2.0+2.3.4-2.3.3+....+n(n+1)+(n+2)-(n-1)+n(n+1)
=>n(n-1)+(n+2)=n(n-1)+(n+2)/3
3A=1.2.3+2.3.3+3.4.3+......+N(N+1).3
3A=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+....+N(N+1)(N-2)-(N-1)
3A=1.2.3-1.2.0+2.3.4-2.3.1+......+N(N+1)+(N+2)-N-1+N(N-1)
=>N(N-1)+(N+2)=N(N-1)-(N+2)/3
tinh tong A= 1.2^2+2.3^2+3.4^2+........+2018.2019^2
tinh
A=1/1.2+1/2.3+1/3.4+.........1/99.100
A=1-1/2+1/2-1/3+1/3-1/4+.........+1/99-1/100
A=1-1/100
A=99/100
ai k mk mk k lai
A = 1 - 1/2 + 1/2 - 1/3 + ......+ 1/99 - 1/100
A = 1 - 1/100
A = 99/100
Ai k mk mk k lại !