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Ta có: \(\frac{9}{56}<\frac{a}{8}<\frac{b}{7}<\frac{13}{28}\)
=> \(\frac{9}{56}<\frac{7a}{56}<\frac{8b}{56}<\frac{26}{56}\)
Nếu \(a=2\)thì \(b=3\)
Ta có : \(\frac{9}{56}<\frac{a}{8}<\frac{b}{7}<\frac{13}{28}\)
=> \(\frac{9}{56}<\frac{7a}{56}<\frac{8b}{56}<\frac{26}{56}\)
=> \(9<7a<8b<26\)
Vì a, b ∈ Z => 7a, 8b ∈ Z
=> 7a, 8b ∈ { 10 ; 11 ; 12 ; 13 ; 14 ; 15 ; 16 ; 17 ; 18 ; 19 ; 20 ; 21 ; 22 ; 23 ; 24 ; 25 }
=> 7a ∈ { 14 ; 21 } ; 8b ∈ { 16 ; 24 }
- Khi 7a = 14 => a = 2
- Khi 7a = 21 => a = 3
- Khi 8b = 16 => b = 2
- Khi 8b = 24 => b = 3
\(S=\frac{3}{2}-\frac{5}{6}+\frac{7}{12}-\frac{9}{20}+\frac{11}{30}-\frac{13}{42}+\frac{15}{56}-\frac{17}{42}\)
\(=1+\frac{1}{2}-\frac{1}{2}-\frac{1}{3}+\frac{1}{3}+\frac{1}{4}-\frac{1}{4}-\frac{1}{5}+\frac{1}{5}\)+ \(\frac{1}{6}-\frac{1}{6}-\frac{1}{7}+\frac{1}{7}\)+ \(\frac{1}{8}-\frac{1}{8}-\frac{1}{9}\)
\(=1-\frac{1}{9}\)
\(=\frac{9}{9}-\frac{1}{9}\)
\(=\frac{8}{9}\)
Chúc bạn học tốt !!!
\(\frac{3}{2}-\frac{5}{6}+\frac{7}{12}-\frac{9}{20}+\frac{11}{30}-\frac{13}{42}+\frac{15}{56}-\frac{17}{72}\)
\(=\frac{3}{1.2}-\frac{5}{2.3}+\frac{7}{3.4}-\frac{9}{4.5}+\frac{11}{5.6}-\frac{13}{6.7}+\frac{15}{7.8}-\frac{17}{8.9}\)
\(=\left(1+\frac{1}{2}\right)-\left(\frac{1}{2}+\frac{1}{3}\right)+\left(\frac{1}{3}+\frac{1}{4}\right)-\left(\frac{1}{4}+\frac{1}{5}\right)+\left(\frac{1}{5}+\frac{1}{6}\right)-\left(\frac{1}{6}+\frac{1}{7}\right)\)
\(+\left(\frac{1}{7}+\frac{1}{8}\right)-\left(\frac{1}{8}+\frac{1}{9}\right)\)
\(=1+\frac{1}{2}-\frac{1}{2}-\frac{1}{3}+\frac{1}{3}+\frac{1}{4}-\frac{1}{4}-\frac{1}{5}+\frac{1}{5}+\frac{1}{6}-\frac{1}{6}-\frac{1}{7}+\frac{1}{7}+\frac{1}{8}-\frac{1}{8}-\frac{1}{9}\)
\(=1-\frac{1}{9}=\frac{8}{9}\)
\(\frac{-9}{56}< \frac{x}{7}< \frac{13}{-28}\Rightarrow\frac{-9}{56}< \frac{8x}{56}< \frac{-26}{56}\Rightarrow-9< 8x< -26\Rightarrow\)vô lí
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