=> 3A = 1.2.3 + 2.3.3 + 3.4.3 + ...+ 123.124.3
3A = 1.2.(3 - 0) + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 123.124.(125 - 122)
3A = 1.2.3 - 1.2.0 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ...+ 123.124.125 - 122.123.124
3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ...+ 123.124.125 - 122.123.124
3A = 123.124.125
=> A = \(\frac{123.124.125}{3}=635500\)
Tính
a) S= 1.2+2.3+3.4+...+32.33
b) S= 1.2+2.3+3.4+...+49.50
Tính nhanh
C=1.2+2.3+3.4+...+2014.2015
K=1.2+2.3+3.4+..+(n-1).n
A= 1.2+2.3+3.4+...........+ 2020.2021
A=1.2+2.3+3.4+...+99.100
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
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 = 1.2+2.3+3.4+.........................+1999.2000
A= 1.2+2.3+3.4+......+ 49.50
A=1.2+2.3+3.4+...+2018.2019
