Tìm \( A = \frac{1}{3 \times 5} + \frac{1}{5 \times 7} + \frac{1}{7 \times 9} + \ldots + \frac{1}{17 \times 19} + \frac{1}{19 \times 21} \)
Bài 1 : Tìm giá trị của A
----- Nội dung ảnh -----
Bài 1: Tìm A
\( A = \frac{1}{3 \times 5} + \frac{1}{5 \times 7} + \frac{1}{7 \times 9} + \ldots + \frac{1}{17 \times 19} + \frac{1}{19 \times 21} \)
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