8/1.5+8/5.9+8/9.15+...+8/97.101
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4\(\frac{1}{2}\)-\(\frac{15}{10.9}-\frac{15}{9.8}-...-\frac{15}{3.2}-\frac{15}{2.1}\)
\(\frac{6}{2.5}+\frac{8}{5.9}+\frac{12}{9.15}\)
\(1-\frac{2}{2.1}-\frac{6}{2.5}-\frac{8}{5.9}-\frac{8}{9.13}\)
Tính :
1.5 + 5.9 + 9.13 + .................. + 97.101
12.A= 1.5.12+5.9(13-1)+9.13(17-5)+13.17(21-9)+.....+97.101(105 - 93)
12.A = 1.5.12 + 5.9.13 -1.5.9 + 9.13.17- 5.9.13 +.....+ 97.101.105 -93.97.101
12.A = 1.5.12 -1.5.9 + 97.101.105
A = (1.5.12 -1.5.9 + 97.101.105):12 = 85725
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Tính :
1.5 + 5.9 + 9.13 + ............. + 97.101
\(\dfrac{4}{3.5}\) + \(\dfrac{8}{5.9}\) + \(\dfrac{12}{9.15}\)+.......+
\(\dfrac{32}{x(x+16)}\) = \(\dfrac{16}{15}\)
\(\dfrac{32}{x(x+16)}\) = \(\dfrac{16}{15}\)
\(\dfrac{HeLp}{me}\) \(\dfrac{nguyễn}{đức}{Trí}\)
\(\dfrac{4}{3.5}+\dfrac{8}{5.9}+\dfrac{12}{9.15}+...+\dfrac{32}{x\left(x+16\right)}=\dfrac{16}{15}\)
\(2.\left(\dfrac{2}{3.5}+\dfrac{4}{5.9}+\dfrac{6}{9.15}+..+\dfrac{16}{X.\left(X+16\right)}\right)=\dfrac{16}{15}\)
\(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{15}+...+\dfrac{1}{X}-\dfrac{1}{X+16}=\dfrac{8}{15}\)
\(\dfrac{1}{X+16}=\dfrac{1}{3}-\dfrac{8}{15}\)
\(\dfrac{1}{X+16}=\dfrac{-1}{5}\)
\(X+16=-5\)
\(X=-21\)
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Tính:
1.5 + 5.9 + 9.13 + 13.17 + ................ + 97.101
1/1.5+1/5.9+1/9.13+...+1/97.101
\(\frac{1}{1.5}+\frac{1}{5.9}+\frac{1}{9.13}+...+\frac{1}{97.101}\)
\(=\frac{1}{4}\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{97}-\frac{1}{101}\right)\)
\(=\frac{1}{4}.\left(1-\frac{1}{101}\right)\)
\(=\frac{1}{4}.\frac{100}{101}\)
\(=\frac{25}{101}\)
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Tính:
1.5 + 5.9 + 9.13 + 13.17 + ................ + 97.101
Tìm số tự nhiên biết: \(\frac{4}{3.5}+\frac{8}{5.9}+\frac{12}{9.15}+...+\frac{32}{n.\left(n+16\right)}=\frac{16}{25}\)
\(\frac{4}{3.5}+\frac{8}{5.9}+\frac{12}{9.15}+...+\frac{32}{n\left(n+16\right)}=\frac{16}{25}\)
\(2\left(\frac{1}{3}-\frac{1}{5}\right)+2\left(\frac{1}{5}-\frac{1}{9}\right)+2\left(\frac{1}{9}-\frac{1}{15}\right)+...+2\left(\frac{1}{n}-\frac{1}{n+16}\right)=\frac{16}{25}\)
\(2\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{15}+...+\frac{1}{n}-\frac{1}{n+16}\right)=\frac{16}{25}\)
\(2\left(\frac{1}{3}-\frac{1}{n+16}\right)=\frac{16}{25}\)
\(\frac{1}{3}-\frac{1}{n+16}=\frac{8}{25}\)
\(\frac{1}{n+16}=\frac{1}{75}\)
\(\Rightarrow n+16=75\)
\(\Rightarrow n=59\)
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bạn Thanh Tùng DZ mình vẫn chưa hiểu tại sao \(\dfrac{8}{5.9}\) = 2.(\(\dfrac{1}{5}\) - \(\dfrac{1}{9}\))
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Tìm x biết :
8/1.5 + 8/5.9 + 8/9.13 +......+ 8/x(x+4) = 1/2
Giúp mình với mình đang cần gấp!!!!!!!!!!!!!!!
\(\frac{8}{1.5}+\frac{8}{5.9}+\frac{8}{9.13}+...+\frac{8}{x\left(x+4\right)}=\frac{1}{2}\)
\(\Leftrightarrow\)\(2\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+...+\frac{4}{x\left(x+4\right)}\right)=\frac{1}{2}\)
\(\Leftrightarrow\)\(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{x}-\frac{1}{x+4}=\frac{1}{4}\)
\(\Leftrightarrow\)\(1-\frac{1}{x+4}=\frac{1}{2}\)
\(\Leftrightarrow\)\(\frac{x+4-1}{x+4}=\frac{1}{2}\)
\(\Leftrightarrow\)\(\frac{x+3}{x+4}=\frac{1}{2}\)
\(\Rightarrow\)\(2\left(x+3\right)=x+4\)
\(\Leftrightarrow\)\(2x+6=x+4\)
\(\Leftrightarrow\)\(x=-2\)
Vậy....
P/s: tham khảo mk ko chắc là đúng
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