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k cho mik nhé

\(\frac{1}{2}+\frac{1}{14}+\frac{1}{35}+\frac{1}{65}+\frac{1}{104}+\frac{1}{152}\)

\(=2.\left(\frac{1}{4}+\frac{1}{28}+\frac{1}{70}+\frac{1}{130}+\frac{1}{208}+\frac{1}{304}\right)\)

\(=2.\left(\frac{1}{1.4}+\frac{1}{4.7}+\frac{1}{7.10}+\frac{1}{10.13}+\frac{1}{13.16}+\frac{1}{16.19}\right)\)

\(=2.\frac{1}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+...+\frac{3}{16.19}\right)\)

\(=\frac{2}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+...+\frac{1}{16}-\frac{1}{19}\right)\)

\(=\frac{2}{3}.\left(1-\frac{1}{19}\right)\)

\(=\frac{2}{3}.\frac{18}{19}=\frac{12}{19}\)

bài này rất dễ nếu để ý nhé

\(\frac{1}{2}+\frac{1}{14}+\frac{1}{35}+\frac{1}{65}+\frac{1}{104}+\frac{1}{152}=\frac{2}{4}+\frac{2}{28}+\frac{2}{70}+\frac{2}{130}+\frac{2}{208}+\frac{2}{304}=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+\frac{2}{10.13}+\frac{2}{13.16}+\frac{2}{16.19}\)rồi phần còn lại em tự tính nhé

1/2 + 1/14 + 1/35 + 1/65 + 1/104 + 1/152  = 12/19 nhé bạn !

de nhu an banh

\(C=\frac{1}{2}+\frac{1}{14}+\frac{1}{35}+\frac{1}{65}+\frac{1}{104}+\frac{1}{152}\)

\(C=\frac{1}{1.2}+\frac{1}{2.7}+\frac{1}{7.5}+\frac{1}{5.13}+\frac{1}{13.8}+\frac{1}{8.19}\)

\(C=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{5}+\frac{1}{5}-\frac{1}{13}+\frac{1}{31}-\frac{1}{8}+\frac{1}{8}-\frac{1}{19}\)

( GẠCH BỎ CÁC PHÂN SỐ GIỐNG NHAU )

\(C=1-\frac{1}{19}\)

\(C=\frac{18}{19}\)

CHÚC BN HỌC TỐT!!!!!!!

Kết quả A = 12/19

\(\Leftrightarrow A=\frac{1}{2}+\frac{1}{2\cdot7}+\frac{1}{7.5}+\frac{1}{5.13}+\frac{1}{13.8}+\frac{1}{8.19}\)

\(\Rightarrow A=\frac{1}{2}+\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{5}+\frac{1}{5}-\frac{1}{13}+\frac{1}{13}-\frac{1}{8}+\frac{1}{8}-\frac{1}{19}\)

\(\Rightarrow A=-\frac{1}{19}\)

hình như là thế ko chắc đâu nhé

\(C=\frac{1}{2}+\frac{1}{14}+\frac{1}{35}+\frac{1}{65}+\frac{1}{104}+\frac{1}{152}\)

\(C=\frac{2}{4}+\frac{2}{28}+\frac{2}{70}+\frac{2}{130}+\frac{2}{208}+\frac{2}{304}\)

\(C=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+\frac{2}{10.13}+\frac{2}{13.16}+\frac{2}{16.19}\)

\(C=\frac{2}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+\frac{3}{13.16}+\frac{3}{16.19}\right)\)

\(C=\frac{2}{3}.\left(3-\frac{3}{4}+\frac{3}{4}-\frac{3}{7}+\frac{3}{7}-\frac{3}{10}+\frac{3}{10}-\frac{3}{13}+\frac{3}{13}-\frac{3}{16}+\frac{3}{16}-\frac{3}{19}\right)\)

\(C=\frac{2}{3}.\left(3-\frac{3}{19}\right)\)

\(C=\frac{2}{3}.\frac{54}{19}\)

\(C=\frac{36}{19}\)

mk sai bước cuối

\(C=\frac{2}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+\frac{1}{13}-\frac{1}{16}+\frac{1}{16}-\frac{1}{19}\right)\)

\(C=\frac{2}{3}.\left(1-\frac{1}{19}\right)\)

\(C=\frac{2}{3}.\frac{18}{19}\)

\(C=\frac{12}{19}\)

C là 12/19 đấy

đơn giản

\(C=\frac{1}{1.2}+\frac{1}{2.7}+\frac{1}{5.7}+\frac{1}{5.13}+\frac{1}{8.13}+\frac{1}{8.19}\)

\(\frac{3}{2}C=\frac{3}{2}\)(\(\frac{1}{1.2}+\frac{1}{2.7}+\frac{1}{5.7}+\frac{1}{5.13}+\frac{1}{8.13}+\frac{1}{8.19}\))

\(\frac{3}{2}C=\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+\frac{3}{13.16}+\frac{3}{16.19}\)

\(\frac{3}{2}C=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+\frac{1}{13}-\frac{1}{16}+\frac{1}{16}-\frac{1}{19}\)

\(\frac{3}{2}C=1-\frac{1}{19}=\frac{18}{19}\)

\(C=\frac{18}{19}:\frac{3}{2}=\frac{12}{19}\)