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\(A=\left(\frac{1}{2}-1\right)\left(\frac{1}{3}-1\right)...\left(\frac{1}{2002}-1\right)\left(\frac{1}{2003}-1\right)\)
\(=\left(-\frac{1}{2}\right)\left(-\frac{2}{3}\right)...\left(-\frac{2001}{2002}\right)\left(-\frac{2002}{2003}\right)\)
\(=\frac{-1.\left(-2\right).....\left(-2001\right)\left(-2002\right)}{2.3....2002.2003}\)
\(=\frac{1}{2003}\)
\(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{2}{x\left(x+1\right)}=\frac{2001}{2003}\)
<=>\(\frac{1}{6}+\frac{1}{12}+...+\frac{1}{x\left(x+1\right)}=\frac{2001}{2003}\cdot\frac{1}{2}=\frac{2001}{4006}\)
<=>\(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{2001}{4006}\)
<=>\(\frac{1}{2}-\frac{1}{x+1}=\frac{2001}{4006}\)
<=>\(\frac{1}{x+1}=\frac{1}{2}-\frac{2001}{4006}\)
<=>\(\frac{1}{x+1}=\frac{1}{2003}\)
<=>x+1=2003
<=>x=2002
a, \(\frac{\left(13\frac{1}{4}-2\frac{5}{27}-10\frac{5}{6}\right).230\frac{1}{25}+46\frac{3}{4}}{\left(1\frac{3}{7}+\frac{10}{3}\right):\left(12\frac{1}{3}-14\frac{2}{7}\right)}\)
\(=\frac{1\frac{25}{108}.320\frac{1}{25}+46\frac{3}{4}}{4\frac{16}{21}:\left(-1\frac{20}{21}\right)}=\frac{330\frac{1}{25}}{-2\frac{18}{41}}=\)\(-135,3164\)
\(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{2}{x\left(x+1\right)}=\frac{2001}{2003}\)
\(2.\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2001}{2003}\)
\(2.\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2001}{2003}\)
\(2.\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2001}{2003}\)
\(2.\left(\frac{1}{2}-\frac{1}{x+1}\right)=\frac{2001}{2003}\)
\(\left(\frac{1}{2}-\frac{1}{x+1}\right)=\frac{2001}{2003}:2\)
\(\frac{1}{2}-\frac{1}{x+1}=\frac{2001}{4006}\)
\(-\frac{1}{x+1}=\frac{2001}{4006}-\frac{1}{2}\)
\(-\frac{1}{x+1}=-\frac{1}{2003}\)
\(\Rightarrow x+1=2003\)
\(\Rightarrow x=2012\)
Ta có: \(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+..+\frac{2}{x\left(x+1\right)}=\frac{2001}{2003}\)
\(\Rightarrow\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+...+\frac{2}{x\left(x+1\right)}=\frac{2001}{2003}\)
\(\Rightarrow2\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2001}{2003}\)
\(\Rightarrow\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{x\left(x+1\right)}=\frac{2001}{2003}:2\)
\(\Rightarrow\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{x\left(x+1\right)}=\frac{2001}{4006}\)
\(\Rightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{2001}{4006}\)
\(\Rightarrow\frac{1}{2}-\frac{1}{x+1}=\frac{2001}{4006}\)
\(\Rightarrow\frac{2003}{4006}-\frac{1}{x+1}=\frac{2001}{4006}\)
\(\Rightarrow\frac{1}{x+1}=\frac{2003}{4006}-\frac{2001}{4006}\)
\(\Rightarrow\frac{1}{x+1}=\frac{2}{4006}=\frac{1}{2003}\)
=> x + 1 = 2003
=> x = 2002
Vậy x = 2002
Duyệt nha !!!
chúc hk tốt!!!
13 +16 +110 +....+1x(x+1):2 =20012003
26 +212 +220 +....+2x(x+1) =20012003
2(12.3 +13.4 +14.5 +....+1x(x+1) )=20012003
12 −13 +13 −14 +14 −15 +....+1x −1x+1 =20012003 :2=20014006
12 −1x+1 =20014006
1x+1 =12 −20014006 =12003
=> x+1 = 2003
=> x = 2003 - 1
=> x = 2002
Xin 1 tích đúng
\(\Rightarrow\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+.....+\frac{2}{x.\left(x+1\right)}=\frac{2001}{2003}\)
\(\Rightarrow2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+....+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2001}{2003}\)
\(\Rightarrow2\left(\frac{1}{2}-\frac{1}{x+1}\right)=\frac{2001}{2003}\)
\(\Rightarrow\frac{x-1}{x+1}=\frac{2001}{2003}\)
\(\Rightarrow2x=4004\)
\(\Rightarrow x=2002\)
Ta có:
1/n + 3 = 1 / 1 + (n + 2)
2/n + 4 = 2 / 2 + (n + 2)
3/n + 5 = 3 / 3 + (n + 2)
....
2001/n + 2003 = 2001 / 2001 + (n + 2)
2002/n + 2004 = 2002 / 2002 + (n + 2)
Ta thấy các phân số trên đều có dạng a/a + (n + 2)
Để mỗi phân số đều tối giản thì a và n + 2 phải nguyên tố cùng nhau
=> n + 2 và 1; 2; 3; ...; 2001; 2002 nguyên tố cùng nhau
Mà n nhỏ nhất => n + 2 nhỏ nhất => n + 2 = 2003
=> n = 2003 - 2 = 2001
Vậy n = 2001
nhớ k nha
Ta có:
1/n + 3 = 1 / 1 + (n + 2)
2/n + 4 = 2 / 2 + (n + 2)
3/n + 5 = 3 / 3 + (n + 2)
....
2001/n + 2003 = 2001 / 2001 + (n + 2)
2002/n + 2004 = 2002 / 2002 + (n + 2)
Ta thấy các phân số trên đều có dạng a/a + (n + 2)
Để mỗi phân số đều tối giản thì a và n + 2 phải nguyên tố cùng nhau
=> n + 2 và 1; 2; 3; ...; 2001; 2002 nguyên tố cùng nhau
Mà n nhỏ nhất => n + 2 nhỏ nhất => n + 2 = 2003
=> n = 2003 - 2 = 2001
Vậy n = 2001
a)\(\left[6.\left(-\frac{1}{3}\right)^2-3\left(-\frac{1}{3}\right)+1\right]:\left(-\frac{1}{3}-1\right)\)
\(=\frac{\left[6\left(-\frac{1}{3}\right)^2+3\left(-\frac{1}{3}\right)+1\right]}{-\frac{1}{3}}-\frac{\left[6\left(-\frac{1}{3}\right)^2-3\left(-\frac{1}{3}\right)+1\right]}{-1}\)
\(=\frac{6\left(-\frac{1}{3}\right)^2}{-\frac{1}{3}}+\frac{3\left(-\frac{1}{3}\right)}{-\frac{1}{3}}-\frac{1}{\frac{1}{3}}+6\left(-\frac{1}{3}\right)^2-3\left(-\frac{1}{3}\right)+1\)
\(=6\left(-\frac{1}{3}\right)+3-3+\frac{6.1}{9}+\frac{3}{3}+1\)
\(=-2+3-3+\frac{2}{3}+1+1=\frac{2}{3}\)
