Đặt \(A=\dfrac{3}{1.2}+\dfrac{3}{2.3}+...+\dfrac{3}{2005.2006}\)
\(A=3\left(\dfrac{1}{1.2}+\dfrac{1}{2.3}+...+\dfrac{1}{2005.2006}\right)\)
\(A=3\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{2005}-\dfrac{1}{2006}\right)\)
\(A=3\left(1-\dfrac{1}{2006}\right)\)
\(A=3.\dfrac{2005}{2006}\)
\(A=\dfrac{6015}{2006}\)
Tính tổng: S= 3/1.2-5/2.3+7/3.4-...-201/100.101
\(\dfrac{3}{1.2}+\dfrac{3}{2.3}+\dfrac{3}{3.4}+...+\dfrac{3}{100.101}\)=?
Tính tổng
A= 1.2+2.3+3.4+...+98.99
B=12+22+32+..+972+982
C=1.99+2.98+3.97+..+97.3+98.2+99.1
Tính tổng sau: A=1.2+2.3+3.4+4.5+5.6+.....+99.100
Tính tổng 1.2+2.3+3.4+...+2015.2016
1) Tính tổng:
A = 1.2 +2.3 + 3.4 + ... + 19.20
2)Tìm số nguyên x, y biết:
xy - x + 2y = 3
Tính
A = \(\frac{1^2}{1.2}.\frac{2^2}{2.3}.\frac{3^2}{3.4}....\frac{1000^2}{1000.1001}\)
CMR: A=3/(1.2)^2+5/(2.3)^2+7/(3.4)^2+...+4033/(2016.2017)^2<1
a)Tính tổng A=1.2+2.3+3.4+...+98.99
Giúp mk nha!Mk cần gấp!