Đặt 2 là thừa số chung rồi tính tổng 1+2+3+4+...+25 ( theo công thức )
\(=2\left(1+2+3+4+...+25\right)\)
\(=2.\dfrac{\left(1+25\right).25}{2}\)
\(=\left(1+25\right).25\)
\(=26.25\)
\(=650\)
Các bạn giúp với :<
Bài 1:
a, CMR: A = \(\dfrac{3}{1^2.2^2}+\dfrac{5}{2^2.3^2}+\dfrac{7}{3^2.4^2}+...+\dfrac{21}{10^2.11^2}< 1\)
b, Cho B = \(\dfrac{3}{4}+\dfrac{8}{9}+\dfrac{15}{16}+\dfrac{24}{25}+...+\dfrac{2499}{2500}.\) CMR: B không phải là số nguyên.
c, So sánh: C = \(\dfrac{2}{2^1}+\dfrac{3}{2^2}+\dfrac{4}{2^3}+...+\dfrac{2021}{2^{2020}}\) với 3.
Tính :
a. 2.3/5 : 2.1/4 + 5/2
b. 16/6 - 2.1/6 : 1.1/7
Tính :
C = \(\left(1+\dfrac{1}{2.3}\right)\) \(\left(1+\dfrac{1}{2.4}\right)\) \(\left(1+\dfrac{1}{3.5}\right)\) .....\(\left(1+\dfrac{1}{2014.2016}\right)\)
tìm x\(\in\)Z biết
1.-10-|5-x|=-12
2.4.|x-5|-54=-34
3.|x-1|+(-5)=2
4.32-8.|2x-9|=24
5.259-51.|4x+3|=(-10)^2.3+10^1
1,Tính
a, A = \(\dfrac{1}{1.2}\) + \(\dfrac{1}{2.3}\) + \(\dfrac{1}{3.4}\) + . . . + \(\dfrac{1}{2017.2018}\)
b,B = \(\dfrac{5}{2.4}\) + \(\dfrac{5}{4.6}\) + \(\dfrac{5}{6.8}\) + . . . + \(\dfrac{5}{2016.2018}\)
c. C = \(\dfrac{1}{18}\) + \(\dfrac{1}{54}\) + \(\dfrac{1}{108}\) + . . . + \(\dfrac{1}{990}\)
1.1+2.2+3.3+.....+1999.1999
1.1+2.2+3.3+.....+1999.1999
âu 5:
Tìm thỏa mãn:
A=1.3+2.4+3.5+...+98.100+99.101
