tính nhanh : 1/2x8+1/8x14+1/14x20+1/20x26+1/26x32
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A = \(\dfrac{1}{1\times2}\) + \(\dfrac{3}{2\times5}\) + \(\dfrac{5}{5\times10}\) + \(\dfrac{4}{10\times14}\) + \(\dfrac{6}{14\times20}\)
A = \(\dfrac{1}{1}\) - \(\dfrac{1}{2}\) + \(\dfrac{1}{2}\) - \(\dfrac{1}{5}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{10}\) + \(\dfrac{1}{10}\) - \(\dfrac{1}{14}\) + \(\dfrac{1}{14}\) - \(\dfrac{1}{20}\)
A = \(\dfrac{1}{1}\) - \(\dfrac{1}{20}\)
A = \(\dfrac{19}{20}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
`B = x^15 - 8x^14 + 8x^13 - 8x^122 + ... - 8x^2 + 8x - 5`
`B = x^15 - 7x^14 -x^14+7x^13+x^13-7x^12-...-x^2+7x+x-5`
`B = x^14(x-7) - x^14(x-7) +...+x^2(x-7)-x(x-7)+x-5`
`B = 7-5=2`
![](https://rs.olm.vn/images/avt/0.png?1311)
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x=7 nên x+1=8
\(B=x^{15}-x^{14}\left(x+1\right)+x^{13}\left(x+1\right)-x^{12}\left(x+1\right)+...-x^2\left(x+1\right)+x\left(x+1\right)-5\)
\(=x^{15}-x^{15}-x^{14}+x^{14}-x^{13}+x^{13}-...-x^3-x^2+x^2+x+5\)
=x+5
=7+5
=12
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\(\dfrac{1}{1.4}+\dfrac{1}{4.2}+\dfrac{1}{2.8}+\dfrac{1}{8.4}+\dfrac{1}{4.16}+\dfrac{1}{16.8}\)
= \(\dfrac{1}{4}.\left(1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}\right)\)
= \(\dfrac{1}{4}.\left(\dfrac{3}{2}+\dfrac{3}{8}+\dfrac{3}{32}\right)\)
= \(\dfrac{1}{4}.\dfrac{3}{2}\left(1+\dfrac{1}{4}+\dfrac{1}{16}\right)\)
= \(\dfrac{1}{4}.\dfrac{3}{2}\left(\dfrac{16}{16}+\dfrac{4}{16}+\dfrac{1}{16}\right)\)
= \(\dfrac{3}{8}.\dfrac{21}{16}\)
= \(\dfrac{63}{128}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
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`1/4 +x=5/2xx8/7`
`=> 1/4 + x= 40/14`
`=> 1/4 + x= 20/7`
`=> x= 20/7-1/4`
`=>x=73/28`
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3+2+2+2.8-1+4.1
=3+2+2+16-1+4
=(3+2+2-1)+(16+4)
=(5+2-1)+20
=(7-1)+20
=6+20
=26