\(=3\cdot2^3\cdot5^2+2\cdot2^6\cdot2^2\cdot3+2^3\cdot2\cdot3\cdot2^2+3\cdot2\cdot3\cdot2^2\\ =2^3\cdot3\cdot5^2+2^9\cdot3+2^6\cdot3+2^3\cdot3^2\\ =2^3\cdot3\left(5^2+2^6+2^3+3\right)=24\left(25+64+8+3\right)\\ =24\cdot100=2400\)

Chọn C

\(B=\left(1-\frac{3}{2.4}\right)\left(1-\frac{3}{3.5}\right)\left(1-\frac{3}{4.6}\right)...\left(1-\frac{3}{n\left(n+2\right)}\right)\)

\(=\frac{1.5}{2.4}.\frac{2.6}{3.5}.\frac{3.7}{4.6}...\frac{\left(n-1\right)\left(n+3\right)}{n\left(n+2\right)}\)

\(=\frac{\left[1.2.3...\left(n-1\right)\right]\left[5.6.7...\left(n+3\right)\right]}{\left(2.3.4...n\right)\left[4.5.6...\left(n+2\right)\right]}\)

\(=\frac{n+3}{4n}< 2\left(đpcm\right)\)

\(1-\dfrac{3}{n\left(n+2\right)}=\dfrac{n\left(n+2\right)-3}{n\left(n+2\right)}=\dfrac{\left(n-1\right)\left(n+3\right)}{n\left(n+2\right)}\)

\(\Rightarrow M=\dfrac{1.5}{2.4}.\dfrac{2.6}{3.5}.\dfrac{3.7}{4.6}...\dfrac{\left(n-1\right)\left(n+3\right)}{n\left(n+2\right)}\)

\(=\dfrac{1.2.3...\left(n-1\right)}{2.3.4...n}.\dfrac{5.6.7...\left(n+3\right)}{4.5.6...\left(n+2\right)}\)

\(=\dfrac{1}{n}.\dfrac{n+3}{4}=\dfrac{n+3}{4n}=\dfrac{1}{4}+\dfrac{3}{4n}>\dfrac{1}{4}\) (đpcm)

Ta có: D\(=\left(1-\dfrac{1}{2}\right)\left(1-\dfrac{1}{3}\right)\left(1-\dfrac{1}{4}\right)...\left(1-\dfrac{1}{2005}\right)\)

\(\Leftrightarrow D=\dfrac{1}{2}.\dfrac{2}{3}.\dfrac{3}{4}...\dfrac{2004}{2005}=\dfrac{1.2.3...2004}{2.3.4...2005}=\dfrac{1}{2005}\)

Ta có: \(E=\dfrac{1^2}{1.3}.\dfrac{2^2}{2.4}.\dfrac{3^2}{3.5}...\dfrac{999^2}{999.1000}.\dfrac{1000^2}{1000.1001}=\dfrac{\left(1.2.3.4...1000\right)\left(1.2.3.4...1000\right)}{\left(1.2.3....1000\right)\left(3.4.5....1001\right)}=\dfrac{2}{1001}\)

Giúp mình nhé !

cảm ơn các bạn !

a, \(x:\left[\left(1800+600\right):30\right]=560:\left(315-35\right)\)

\(\Rightarrow\) \(x:\left[2400:30\right]=560:280\)

\(\Rightarrow\) \(x:80=2\)

\(\Rightarrow\) \(x=160\)

b, \(\left[\left(250-25\right):15\right]:x=\left(450-60\right):130\)

\(\Rightarrow\) \(\left[225:15\right]:x=390:130\)

\(\Rightarrow\) \(15:x=3\)

\(\Rightarrow\) \(x=5\)

https://dethihsg.com/de-thi-hoc-sinh-gioi-phong-gddt-hoang-hoa-2014-2015/

vào đây gợi ý nhé

k mik đi

@_@

đây nè

k mik nha