Bài 1 :
a) \(\frac{1}{1.2}\)+ \(\frac{1}{2.3}\)+.....+ \(\frac{1}{a\left(a+1\right)}\)
b) \(\frac{1}{1.2.3}+\frac{1}{2.3.4}+.....+\frac{1}{a\left(a+1\right).\left(a+2\right)}\)
Những câu hỏi liên quan
Tính :
a) (1+2+3+..+2017+2018).(\(\frac{3}{1.2}+\frac{3}{2.3}+...+\frac{3}{2018.2019}\)) . \(\left(\frac{1}{2}-\frac{1}{3}-\frac{1}{6}\right)\)
b) A= \(1.2.3+2.3.4+3.4.5+...+x.\left(x+1\right).\left(x+2\right)\)
a)Xét 1/2-1/3-1/6=3/6-2/6-1/6=0
=> (1+2+3+...+2018).(3/1.2+3/2.3+...+3/2018.2019).(1/2-1/3-1/6)=(1+2+3+...+2018).(3/1.2+3/2.3+...+3/2018.2019).0=0
b) 4A=1.2.3.4+2.3.4.4+..+x(x+1)(x+2)4
=1.2.3.4+2.3.4.5-1.2.3.4+...+x(x+1)(x+2)(x+3)-x(x+1)(x+2)(x-1)
= (x-1)x(x+1)(x+2)
=> A=x(x+1)(x+2)(x-1)/4
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help me! (ngu toàn tập)a)frac{3}{left(1.2right)^2}+frac{5}{left(2.3right)^2}+...+frac{2n+1}{left[nleft(n+1right)right]^2}b)left(1-frac{1}{2^2}right)left(1-frac{1}{3^2}right)left(1-frac{1}{4^2}right)....left(1-frac{1}{n^2}right)c)frac{150}{5.8}+frac{150}{8.11}+frac{150}{11.14}+...+frac{150}{47.50}d)frac{1}{1.2.3}+frac{1}{2.3.4}+frac{1}{3.4.5}+...+frac{1}{left(n-1right)nleft(n+1right)}
Đọc tiếp
help me! (ngu toàn tập)
a)\(\frac{3}{\left(1.2\right)^2}+\frac{5}{\left(2.3\right)^2}+...+\frac{2n+1}{\left[n\left(n+1\right)\right]^2}\)
b)\(\left(1-\frac{1}{2^2}\right)\left(1-\frac{1}{3^2}\right)\left(1-\frac{1}{4^2}\right)....\left(1-\frac{1}{n^2}\right)\)
c)\(\frac{150}{5.8}+\frac{150}{8.11}+\frac{150}{11.14}+...+\frac{150}{47.50}\)
d)\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{\left(n-1\right)n\left(n+1\right)}\)
\(\frac{150}{5.8}+\frac{150}{8.11}+\frac{150}{11.14}+.....+\frac{150}{47.50}\)
\(=50.\left(\frac{3}{5.8}+\frac{5}{8.11}+.....+\frac{3}{47.50}\right)\)
\(=50.\left(\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+......+\frac{1}{47}-\frac{1}{50}\right)\)
\(=50.\left(\frac{1}{5}-\frac{1}{50}\right)\)
\(=50.\frac{9}{50}=9\)
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Tính các tổng :a) Afrac{1}{1.2}+frac{1}{2.3}+frac{1}{3.4}+...+frac{1}{nleft(n+1right)} ( Hướng dẫn : frac{1}{kleft(k+1right)}frac{1}{k}-frac{1}{k+1})b) Bfrac{1}{1.2.3}+frac{1}{2.3.4}+frac{1}{3.4.5}+...+frac{1}{nleft(n+1right)left(n+2right)} ( Hướng dẫn : frac{1}{kleft(k+1right)left(k+2right)}frac{1}{2}left(frac{1}{k}+frac{1}{k+2}right)-frac{1}{k+1})
Đọc tiếp
Tính các tổng :
a) \(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{n\left(n+1\right)}\) ( Hướng dẫn : \(\frac{1}{k\left(k+1\right)}=\frac{1}{k}-\frac{1}{k+1}\))
b) \(B=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{n\left(n+1\right)\left(n+2\right)}\)
( Hướng dẫn : \(\frac{1}{k\left(k+1\right)\left(k+2\right)}=\frac{1}{2}\left(\frac{1}{k}+\frac{1}{k+2}\right)-\frac{1}{k+1}\))
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{n\left(n+1\right)}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{n}-\frac{1}{n+1}\)
\(A=1-\frac{1}{n+1}\)
a) Ta có: \(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{n\left(n+1\right)}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{n}-\frac{1}{n+1}\)
\(A=1-\frac{1}{n+1}\)
\(A=\frac{n+1}{n+1}-\frac{1}{n+1}\)
\(A=\frac{n}{n+1}\)
Học tốt nha^^
\(B=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{n\left(n+1\right)\left(n+2\right)}\)
\(\Rightarrow2B=\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{n\left(n+1\right)\left(n+2\right)}\)
\(\Rightarrow2B=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\)
\(\frac{1}{n\left(n+1\right)}-\frac{1}{\left(n+1\right)\left(n+2\right)}\)
\(\Rightarrow2B=\frac{1}{1.2}-\frac{1}{\left(n+1\right)\left(n+2\right)}\)
\(\Rightarrow B=\frac{1}{4}-\frac{1}{2\left(n+1\right)\left(n+2\right)}\)
tính: \(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{a\left(a+1\right)\left(a+2\right)}\)
Tìm x biết
\(\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{2013.2014.2015}\right)x=\left(1.2+2.3+3.4+.....+2014.2015\right)\)
\(\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+....+\frac{1}{2013.2014}-\frac{1}{2014.2015}\right)x=\frac{1}{3}\left(2014.2015.2016-2013.2014.2015........+2.3.4-1.2.3+1.2.3-0.1.2\right)\)
\(\left(\frac{1}{2}-\frac{1}{2014.2015}\right)x=\frac{1}{3}.2014.2015.2016\)
\(x=\frac{1}{3.2029104}.2014^2.2015^2.2016=\)
\(\left(\frac{1}{2}-\frac{1}{2014.2015}\right)x=\frac{1}{3}.2014.2015.2016\)
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Tính A = \(\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\right)-\left(\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{97.99}\right)+\left(-2-4-6-...-100\right)+\)\(\left(-1.2-2.3-3.4-...-99.100\right)\)
\(F=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{\left(n-1\right)n}=\frac{n-1}{n}\)
\(G=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{\left(n-1\right)\left(n-1\right).n}=\)
\(H=2+4+6+..+2n=\)
\(F=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{\left(n-1\right)n}=\frac{n-1}{n}\)
\(\Rightarrow F=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{\left(n-1\right)}-\frac{1}{n}\)
\(\Rightarrow F=1-\frac{1}{n}=\frac{n}{n}-\frac{1}{n}=\frac{n-1}{n}\left(đpcm\right)\)
\(H=2+4+6+...+2n\)
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giải phương trình:
\(\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{2005.2006.2007}\right).x=\left(1.2+2.3+3.4+...+2006.2007\right)\)
ta đặt: A = 1/1.2.3 + 1/2.3.4 + 1/3.4.5 +...+ 1/2005.2006.2007
2.A = 2(1/1.2.3 + 1/2.3.4 + 1/3.4.5 +...+ 1/2005.2006.2007)
2.A = 2/1.2.3 + 2/2.3.4 + 2/3.4.5 +...+ 2/2005.2006.2007
= (1/1.2 - 1/2.3) + (1/2.3 - 1/3.4) +...+ (1/2005.2006- 1/2006.2007)
= 1/1.2 - 1/2.3 + 1/2.3 - 1/3.4 + ... +1/2005.2006 - 1/2006.2007
= 1/1.2 - 1/2006.2007
=> A = (1/1.2 - 1/2006.2007):2
A = 1/4 - 1/1003.2007
Đặt B = 1/1.2 + 1/2.3+ 1/ 3.4 ..... + 1/2006.2007
=(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+....+(1/2006-1/2007)
=1/1-1/2+1/2-1/3+1/3-1/4+....+1/2006-1/2007
=1/1-1/2007
= 2006/2007
thay vào phương trình ta có phương trình trở thành:
(1/4 - 1/1003.2007).x = 2006/2007
..........
còn lại bạn tính nhé
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Giải pt \(\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{2005.2006.2007}\right)x=\left(1.2+2.3+...+2006.2007\right)\)
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