Ta có : \(B=\frac{4}{3.4}-\frac{4}{4.5}-\frac{4}{5.6}-.......-\frac{4}{9.10}\)
\(\Rightarrow B=\frac{4}{3.4}-\left(\frac{4}{4.5}+\frac{4}{5.6}+\frac{4}{6.7}+......+\frac{4}{9.10}\right)\)
\(\Rightarrow B=\frac{1}{3}-4\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+.....+\frac{1}{9.10}\right)\)
\(\Rightarrow B=\frac{1}{3}-4.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+......+\frac{1}{9}-\frac{1}{10}\right)\)
\(\Rightarrow B=\frac{1}{3}-4.\left(\frac{1}{4}-\frac{1}{10}\right)\)
\(\Rightarrow B=\frac{1}{3}-\frac{3}{5}=-\frac{4}{15}\)
B = 4 x ( 1/3.4 - 1/4.5 -...-1/9.10)
B = 4 x [1/3.4 - (1/4.5 + 1/5.6 + ... + 1/9.10)]
B = 4 x [1/3.4 - (1/4 - 1/5 + 1/5 - 1/6 + ... + 1/9 - 1/10)]
B = 4 x [1/12 - (1/4 - 1/10)]
B = 4 x (1/12 - 3/20)
B = 4 x (5/60 - 9/60)
B = 4 x -4/60
B = 4 x -1/15
B = -4/15
k cho mình nếu đúng nhé
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