S = 1.2 + 2.3 + 3.4 + ... + 999.1000
<=> 3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + 999.1000.3
xét 3.n.(n + 1)
= 3n.(n + 1)
= n.(n + 1)(n + 2 - n + 1)
= n.(n + 1)(n + 2) - n(n - 1)(n + 1)
thay vào S được
3S = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 999.1000.1001 - 998.999.1000
=> S = 999.1000.1001 ÷ 3 = 333333000
1/1.2+1/2.3+1/3.4+......+1/999.1000
Tính tổng : S=2.3+3.4+4.5+...+999.1000
tính
a, 1/1.2+1/2.3+1/3.4+....+1/999.1000
b, B= 1/2.4+1/4.6+1/6.8+1/8.10
12 /1.2 . 22/2.3 . 32/3.4 ... 9992/999.1000
Tính:
a) A = \(\dfrac{1}{1.2}\) + \(\dfrac{1}{2.3}\) + \(\dfrac{1}{3.4}\) +...+ \(\dfrac{1}{998.999}\) + \(\dfrac{1}{999.1000}\)
b) B = \(\dfrac{1}{1.6}\) + \(\dfrac{1}{6.11}\) + \(\dfrac{1}{11.16}\) +...+ \(\dfrac{1}{495.500}\)
c) C = \(\dfrac{1}{1.2.3}\) + \(\dfrac{1}{2.3.4}\) + \(\dfrac{1}{3.4.5}\) +...+ \(\dfrac{1}{998.999.1000}\)
(Mong mn giúp ạ)
Tính
a) S= 1.2+2.3+3.4+...+32.33
b) S= 1.2+2.3+3.4+...+49.50
1 . Tính : 20 ^2 + 22^2 + 24^2 + ...... + 48 ^2 + 50^2
2 . Cho N thuộc N * . Tính tổng
n^2 + ( n +2 ) ^2 + ( n + 4 )^2 + ......... + n +100 ^ 2
3 . Tính : 1.2 + 2.3 + 3.4 + 4.5 + ......... + 999.1000
(1.2)2 / ( 2.3)2 . (3.4)2/ (4.5)2 .... (999.1000)2 / (1000.1001)2
Tính tổng: S = 1.2 + 2.3 + 3.4 + 4.5 + ... + 99.100.
