A=1.2+2.3+....+2014.2015
=>3A=1.2.3+2.3.3+3.4.3+...+2015.2014.3
=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+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)+....+(2013.2014.2015-2013.2014.2015)+0.1.2+2013.2014.2015
=0+2013.2014.2015
=>A=\(\frac{2013.2014.2015}{3}\)
Tính nhanh
C=1.2+2.3+3.4+...+2014.2015
K=1.2+2.3+3.4+..+(n-1).n
1.2 + 2.3 + 3.4 +...+2014.2015
tinh tong A=1.2+2.3+3.4+4.5+...+2014.2015
4/1.2+4/2.3+4/3.4+...+4/2014.2015
D=1(1+2+3+......+2014)+(1.2+2.3+3.4+.....+2014.2015)
Tinh tong S =1/1.2+1/2.3+1/3.4+...+1/2014.2015+1/2015.2016
tính:
a)\(A=\frac{4}{1.2}+\frac{4}{2.3}+\frac{4}{3.4}+...+\frac{4}{2014.2015}\)
(còn nữa)
Giúp mình bài này
A=4/1.2+4/2.3+4/3.4+..........+4/2014.2015
tính giá trị biểu thức : \(\dfrac{4}{1.2}\)+\(\dfrac{4}{2.3}\)+\(\dfrac{4}{3.4}\)+ ... + \(\dfrac{4}{2014.2015}\)
