Bài 2:
a: \(A=\dfrac{3}{1\cdot4}+\dfrac{3}{4\cdot7}+...+\dfrac{3}{40\cdot43}\)
\(=1-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+...+\dfrac{1}{40}-\dfrac{1}{43}\)
=1-1/43<1
b: \(B=2\left(\dfrac{3}{2\cdot5}+\dfrac{3}{5\cdot8}+...+\dfrac{3}{29\cdot32}\right)\)
\(=2\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+...+\dfrac{1}{29}-\dfrac{1}{32}\right)\)
\(=2\cdot\dfrac{16-1}{32}=2\cdot\dfrac{15}{32}=\dfrac{15}{16}< 1\)
Đúng 2
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