Tim n thuộc N
A = \(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+...+\frac{2}{n\times\left(n+2\right)}<\frac{2015}{2016}\)
Những câu hỏi liên quan
A = \(\left(1+\frac{1}{1\times3}\right)\times\left(1+\frac{1}{2\times4}\right)\times\left(1+\frac{1}{3\times5}\right)\times....\times\left(1+\frac{1}{5\times7}\right)\)=?
1=3/3=4/4=5/5=...
=> 1+1/1*3=3/1*3=1/1
=> 1+1/2*4=4/2*4=1/2
=>...
Bieu thuc se con lai la 1*1/2*1/3*1/4*1/5
Vay A=1/120
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Tìm x, biết:
Xem chi tiết
\(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+.....+\frac{2}{x\times\left(x+2\right)}=\frac{2015}{2016}\)
Bài làm
\(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+.....+\frac{2}{x.\left(x+2\right)}=\frac{2015}{2016}\)
\(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+.....+\frac{1}{x}-\frac{1}{x+2}=\frac{2015}{2016}\)
\(1-\frac{1}{x+2}=\frac{2015}{2016}\)
\(\frac{1}{x+2}=\frac{1}{2016}\)
\(\Rightarrow x+2=2016\)
\(x=2014\)
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Phương trình
<=>\(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{x}-\frac{1}{x+2}=\frac{2015}{2016}\)
<=> \(1-\frac{1}{x+2}=\frac{2015}{2016}\)
=> x=2014
Vậy x=2014
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\(\left(\frac{1}{1\times3}+\frac{1}{3\times5}+\frac{1}{5\times7}+\frac{1}{7\times9}+\frac{1}{9\times11}\right)\times y=\frac{2}{3}\)
Tìm y
\(\frac{1}{2}\times\left(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+\frac{2}{9\times11}\right)\times y=\frac{2}{3}\)
\(\frac{1}{2}\times\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right)\times y=\frac{2}{3}\)
\(\frac{1}{2}\times\left(\frac{1}{1}-\frac{1}{11}\right)\times y=\frac{2}{3}\)
\(\frac{1}{2}\times\frac{10}{11}\times y=\frac{2}{3}\)
\(\frac{5}{11}\times y=\frac{2}{3}\) => \(y=\frac{2}{3}:\frac{5}{11}=\frac{2}{3}\times\frac{11}{5}=\frac{22}{15}\)
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frac{left(-7right)^n}{left(-7right)^{n-1}}(nge1) Tính GTBT Bài 2 Tính GTBT theo cách hợp lí nếu có thểc) frac{5^3times3^3}{5^3times0,5+125times2,5}d)frac{5times7^1+7^3times25}{7^5125-7^3times50}e)frac{8^5timesleft(-5right)^8+left(-2right)^5times10^9}{2^{16}times5^7+20^8}h)frac{left(-0,25right)^{-5}times9^4timesleft(-2right)^{-3}-2^{-2}times6^3}{2^9times3^6+6^6times40}Bài 3 Chứng tỏ rằnga)
Đọc tiếp
\(\frac{\left(-7\right)^n}{\left(-7\right)^{n-1}}\)(n\(\ge1\)) Tính GTBT
Bài 2 Tính GTBT theo cách hợp lí nếu có thể
c) \(\frac{5^3\times3^3}{5^3\times0,5+125\times2,5}\)d)\(\frac{5\times7^1+7^3\times25}{7^5125-7^3\times50}\)e)\(\frac{8^5\times\left(-5\right)^8+\left(-2\right)^5\times10^9}{2^{16}\times5^7+20^8}\)
h)\(\frac{\left(-0,25\right)^{-5}\times9^4\times\left(-2\right)^{-3}-2^{-2}\times6^3}{2^9\times3^6+6^6\times40}\)
Bài 3 Chứng tỏ rằng
a)
Tính C=\(\frac{1}{1\times2\times3}+\frac{1}{2\times3\times4}+\frac{1}{3\times4\times5}+....+\frac{1}{n\times\left(n+1\right)\times\left(n+2\right)}\)
Bạn nào giúp mik nhớ viết cả cách giải cho mik nhé!!!!!!!!!!
a,\(\frac{3^{10}\times(-5)^{21}}{\left(-5\right)^{20}\times3^{12}}\)
b,\(\frac{\left(-11\right)^5\times13^7}{11^5\times13^8}\)
c,\(\frac{2^{10}\times3^{10}-2^{10}\times3^9}{2^9\times3^{10}}\)
d,\(\frac{5^{11}\times7^{12}+5^{11}\times7^{11}}{5^{12}\times7^{12}+9\times5^{11}\times7^{11}}\)
Bài trên là bài rút gon phân số
\(a)\frac{3^{10}.\left(-5\right)^{21}}{\left(-5\right)^{20}.3^{12}}=\frac{-5}{3^2}=\frac{-5}{9}\)
\(b)\frac{-11.13^7}{11^5.13^8}=\frac{-1}{11^4.13}\) (Bạn xem thử xem có sai đề không nhé)
\(c)\frac{2^{10}.3^{10}-2^{10}.3^9}{2^9.3^{10}}=\frac{2^{10}.3^9\left(3+1\right)}{2^9.3^{10}}=\frac{2.4}{3}=\frac{8}{3}\)
\(d)\frac{5^{11}.7^{12}+5^{11}.7^{11}}{5^{12}.7^{12}+9.5^{11}.7^{11}}=\frac{5^{11}.7^{11}\left(7+1\right)}{5^{11}.7^{11}\left(5.4+9\right)}=\frac{8}{20+9}=\frac{8}{29}\)
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\(a)\frac{3^{10}\cdot\left(-5\right)^{21}}{\left(-5\right)^{20}\cdot3^{12}}=\frac{-5}{3^2}=\frac{-5}{9}\)
\(b)\frac{\left(-11\right)\cdot13^7}{11^5\cdot13^8}=\frac{-1}{11^4\cdot13}=\frac{-1}{14641\cdot13}=\frac{-1}{190333}\)
\(c)\frac{2^{10}\cdot3^{10}-2^{10}\cdot3^9}{2^9\cdot3^{10}}=\frac{2^{10}\left(3^{10}-3^9\right)}{2^9\cdot3^{10}}=\frac{2^{10}\cdot3^9\left(3-1\right)}{2^9\cdot3^{10}}=\frac{2^{10}\cdot3^9\cdot2}{2^9\cdot3^{10}}=\frac{2\cdot2}{3}=\frac{4}{3}\)
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Tính:
\(\frac{exp\left(24\right)}{\frac{25}{2}\%}\times12\frac{5}{4}+\left|-54^2\right|\times\pi+6\)\(\times7\sqrt{65}+\frac{7\times7^2\times7^3\times7^4}{3\times3^2\times3^3\times3^4}\)
Rut gon phan so sau :
a)\(\frac{9^{14\times}25^5\times8^7}{18^{12}\times625^3\times24^3}\)
b)\(\frac{1\times3\times5\times...\times39}{21\times22\times23\times...\times40}\)
c)\(\frac{1\times3\times5\times...\times\left(2n-1\right)}{\left(n+1\right)\times\left(n+2\right)\times\left(n+3\right)\times...\times2n}\)
\(\frac{9^{14}.25^5.8^7}{18^{12}.625^3.24^3}=\frac{9^{12}.9^2.25^5.8^3.8^5}{9^{12}.2^{12}.25^6.8^3.3^3} =\frac{3^4.8^5 }{8^4.3^3}=3.8=24\)
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CMR với mọi số tự nhiên n lớn hơn hoặc bằng 1 thì:
\(\left(1+\frac{1}{1\times3}\right)\left(1+\frac{1}{2\times4}\right)\left(1+\frac{1}{3\times5}\right).......\left(1+\frac{1}{n\times\left(n+2\right)}\right)< 2\)