Question

[Phạm Nguyễn Hùng Nguyên]

4/1.4+4/4.7+...+4/25.28

Answer 1

[Nguyễn Ngọc Anh Minh]

\(\frac{3A}{4}=\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{25.28}.\)

\(\frac{3A}{4}=\frac{4-1}{1.4}+\frac{7-4}{4.7}+\frac{10-7}{7.10}+...+\frac{28-25}{25.28}\)

\(\frac{3A}{4}=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{25}-\frac{1}{28}=1-\frac{1}{28}=\frac{27}{28}\)

\(A=\frac{27.4}{28.3}=\frac{9}{7}\)

Answer 2

[Phạm Nguyễn Hùng Nguyên]

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