2

\(A=9\left(\frac{1}{4x5}+\frac{1}{5x6}+\frac{1}{6x7}+...+\frac{1}{34x35}+\frac{1}{35.36}\right)\)

\(A=9\left(\frac{5-4}{4x5}+\frac{6-5}{5x6}+\frac{7-6}{6x7}+...+\frac{35-34}{34x35}+\frac{36-35}{35x36}\right)\)

\(A=9\left(\frac{1}{4}-\frac{1}{36}\right)=9x\frac{8}{36}=2\)

Toán lớp 5 à ?

Sao khó quá vậy ?

Nhìn là ngán chả mún làm ! phù...............

Mí bạn k nha !

2 là đáp án đúng

\(\frac{9}{4.5}+\frac{9}{5.6}+...+\frac{9}{35.36}\)

\(9.\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{35.36}\right)\)

\(9.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{35}-\frac{1}{36}\right)\)

\(9.\left(\frac{1}{4}-\frac{1}{36}\right)\)

\(9.\frac{2}{9}\)

\(=2\)

Bằng 2 nhé bạn

giup minh voi mn oi

A=  \(\frac{221}{210}\)

k tui nha, hậu tạ

sao mày o làm nhanh ả

không nói tục chửi bậy nha

\(\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{7\times8}+....+\dfrac{1}{9\times10}\)

\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+.....+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=\dfrac{1}{3}-\dfrac{1}{10}\)

\(=\dfrac{7}{30}\)

Sửa đề:

\(\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+\dfrac{1}{8\times9}+\dfrac{1}{9\times10}\)

\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=\dfrac{1}{3}-\dfrac{1}{10}\)

\(=\dfrac{10}{30}-\dfrac{3}{30}\)

\(=\dfrac{7}{30}\)

\(\dfrac{1}{3x4}+\dfrac{1}{4x5}+\dfrac{1}{5x6}+\dfrac{1}{6x7}+\dfrac{1}{7x8}+\dfrac{1}{8x9}+\dfrac{1}{9x10}\)

\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=\dfrac{1}{3}-\dfrac{1}{10}\)

\(=\dfrac{10}{30}-\dfrac{3}{30}\)

\(=\dfrac{7}{30}\)

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