A=1.2+2.3+3.4+...+123.124= ?
Tính
a) S= 1.2+2.3+3.4+...+32.33
b) S= 1.2+2.3+3.4+...+49.50
Ta có : S = 1.2 + 2.3 + 3.4 + ..... + 32.33
=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ...... + 32.33.34
=> 3S = 32.33.34
=> S = \(\frac{32.33.34}{3}=11968\)
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
Tính nhanh
C=1.2+2.3+3.4+...+2014.2015
K=1.2+2.3+3.4+..+(n-1).n
cau hỏi tương tự ko có mà!!!!!!!!!!!!!!!!!!!!!!!!!!!!
3C=1.2.3+2.3.(4-1)+3.4.(5-2)+...+2014.2015.(2016-2013)
3C=2014.2015.2016
C=2014.2015.2016:3
A= 1.2+2.3+3.4+...........+ 2020.2021
A=1.2+2.3+3.4+...+99.100
3S=1*2*3+2*3*(4-1)+...+99*100*(101-98)
=1*2*3+2*3*4-1*2*3+...+99*100*101-99*100*98
=99*100*101
=>S=33*100*101=333300
Tính các biểu thức sau:
C=1.2+2.3+3.4+...+98.99
D=(1.99+2.99+3.99+...99.99)-(1.2+2.3+3.4+...98.99
C=1*2+2*3+3*4+...+98*99
C=2+6+12+...+9702
C=2+9702
C=9704
vay C=9704
D=(1*99+2*99+3*99+...+99*99)-(1*2+2*3+3*4+...+98*99)
D=(99+198+297+...+9801)-(2+6+12+...+9702)
D=(99+9801)-(2+9702)
D=9900-9704
D=196
vay D=196
ai di qua dong tinh thi nho h cho minh nhe
cho A = 1/1.2+1/2.3+1/3.4+...+1/49.50 ; cho B = 1.2+1.3+3.4+....+49.50
tính 50mủ 2A - B/17
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{49.50}\)
\(=\frac{2-1}{1.2}+\frac{3-2}{2.3}+\frac{4-3}{3.4}+...+\frac{50-49}{49.50}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{49}-\frac{1}{50}\)
\(=1-\frac{1}{50}=\frac{49}{50}\)
\(B=1.2+2.3+3.4+...+49.50\)
\(3B=1.2.3+2.3.3+3.4.3+...+49.50.3\)
\(=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+49.50.\left(51-48\right)\)
\(=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+49.50.51-48.49.50\)
\(=49.50.51\)
\(B=\frac{49.50.51}{3}=49.50.17\)
\(50^2.A-\frac{B}{17}=49.50-49.50=0\)
A = 1.2+2.3+3.4+.........................+1999.2000
A = 1.2+2.3+3.4+.........................+1999.2000
=> 3A= 1.2.(3-0) + 2.3.(4-1) + ... + 1999.2000(2001-1998)
=> 3A=1.2.3+2.3.4-1.2.3+...+1999.2000.2001-1998.1999.2000
=> 3A=1999.2000.2001
A=1999.2000.2001:3=2666666000
A= 1.2+2.3+3.4+......+ 49.50
A = 1.2 + 2.3 + ........+49.50
3A = 1.2.(3-0) + 2.3.(4-1)+........+49.50.(51 - 48)
3A = 1.2.3 + 2.3.4 - 1.2.3 +........ + 49.50.51 - 48.49.50
3A = 48.49.50 = 117600
A = 39200
Ta có :
Gọi A=1.2+2.3+3.4+4.5+...+49.50
A=1.2+2.3+3.4+4.5+...+49.50
3.A=3.(1.2+2.3+3.4+4.5+...+49.50)
3.A=1.2.3+2.3.3+3.3.4+3.4.5+...+3.49.50
3.A=1.2.(3-0)+2.3.(3-0)+(3-0).3.4+(3-0).4.5+...+(3-0).49.50
3.A=1.2.3-0+2.3.3-0+3.3.4-0+3.4.5-0+...+3.49.50-0
3.A=1.2.3-0+2.3.4-1.2.3+5.3.4-2.3.4+...+49.50.51-48.49.50
3.A=49.50.51
A= 49.50.51/3
A= 49.50.17.3/3
A=49.50.17
A=41650
Đáp số : A=41650
1x2+2x3+3x4+4x5+...+49x50=A
3.(1.2+2.3+3.4+4.5+...+49.50)=A.3
3.1.2+3.2.3+3.3.4+3.4.5+...+3.49.50+A.3
(3-0).1.2+(4-1).2.3+(5-2).3.4+(6-3).4.5+...+(51-48).49.50=A.3
3.1.2-0.1.2+4.2.3-1.2.3+5.3.4-2.3.4+6.4.5-3.4.5+...+51.49.50-48,49,50=A.3
51.49.50=A.3
51.49.50:3=A
các bạn cứ tính kết quả ra là ok