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\(A=\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}+\dfrac{1}{7\cdot8}\)
\(=\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{7}-\dfrac{1}{8}=\dfrac{3}{8}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
A = \(\dfrac{x+13}{x+2}\) (đk \(x\) ≠ -2)
Em cần làm gì với biểu thức này?
![](https://rs.olm.vn/images/avt/0.png?1311)
C={7;8;9;10;11}
D={0;2;4;6;8}
C có 5 phần tử
D có 5 phần tử
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\(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{99\cdot100}\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{99}-\dfrac{1}{100}\)
\(=1-\dfrac{1}{100}=\dfrac{99}{100}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
n.m=36
=> n.(-m)= n.m.(-1)=36.(-1)= -36
(-n).(-m)= n.m. (-1). (-1)= n.m.1= 36.1=36
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s: \(\dfrac{-21}{46}\cdot\left(-13\right)+\dfrac{3^2}{-9}-\dfrac{-1}{2}\cdot\left(-10\right)\)
\(=\dfrac{21}{46}\cdot13-1-\dfrac{1}{2}\cdot10\)
\(=\dfrac{273}{46}-1-5=\dfrac{273}{46}-5=\dfrac{43}{46}\)
t: \(T=\left(-\dfrac{1}{7}\right)+\left(-\dfrac{1}{7}\right)^2+...+\left(-\dfrac{1}{7}\right)^{2024}\)
=>\(\left(-\dfrac{1}{7}\right)\cdot T=\left(-\dfrac{1}{7}\right)^2+\left(-\dfrac{1}{7}\right)^3+...+\left(-\dfrac{1}{7}\right)^{2025}\)
=>\(\left(-\dfrac{1}{7}\right)\cdot T-T=\left(-\dfrac{1}{7}\right)^2+\left(-\dfrac{1}{7}\right)^3+...+\left(-\dfrac{1}{7}\right)^{2025}-\left(-\dfrac{1}{7}\right)-\left(-\dfrac{1}{7}\right)^2-...-\left(-\dfrac{1}{7}\right)^{2024}\)
=>\(-\dfrac{8}{7}T=\left(-\dfrac{1}{7}\right)^{2025}+\dfrac{1}{7}\)
=>\(-\dfrac{8}{7}\cdot T=-\dfrac{1}{7^{2025}}+\dfrac{1}{7}\)
=>\(-\dfrac{8}{7}\cdot T=\dfrac{-1+7^{2024}}{7^{2025}}\)
=>\(T\cdot\dfrac{8}{7}=\dfrac{-7^{2024}+1}{7^{2025}}\)
=>\(T=\dfrac{-7^{2024}+1}{7^{2025}}:\dfrac{8}{7}=\dfrac{-7^{2024}+1}{7^{2024}}\cdot8\)
u: \(U=\dfrac{1}{5}-\dfrac{1}{5^2}+\dfrac{1}{5^3}-...-\dfrac{1}{5^{2024}}\)
=>\(5\cdot U=1-\dfrac{1}{5}+\dfrac{1}{5^2}-...-\dfrac{1}{5^{2023}}\)
=>\(5U+U=1-\dfrac{1}{5}+\dfrac{1}{5^2}-...-\dfrac{1}{5^{2023}}+\dfrac{1}{5}-\dfrac{1}{5^2}+...-\dfrac{1}{5^{2024}}\)
=>\(6U=1-\dfrac{1}{5^{2024}}=\dfrac{5^{2024}-1}{5^{2024}}\)
=>\(U=\dfrac{5^{2024}-1}{5^{2024}\cdot6}\)