1. Tính nhanh: \(\frac{1}{2\times4}+\frac{1}{4\times6}+\frac{1}{6\times8}+....+\frac{1}{\overline{ }2016\times2018}\)2. Tìm x, biết:a. X x \(\frac{2}{7}\)x \(\frac{3}{4}\)= \(\frac{5}{21}\) b. \(\frac{17}{5}\): X = \(\frac{34}{5}\): \(\frac{4}{3}\) c. X...
Đọc tiếp
1. Tính nhanh:
\(\frac{1}{2\times4}+\frac{1}{4\times6}+\frac{1}{6\times8}+....+\frac{1}{\overline{ }2016\times2018}\)
2. Tìm x, biết:
a. X x \(\frac{2}{7}\)x \(\frac{3}{4}\)= \(\frac{5}{21}\) b. \(\frac{17}{5}\): X = \(\frac{34}{5}\): \(\frac{4}{3}\) c. X : \(\frac{3}{4}\)= \(\frac{28}{9}\): \(\frac{4}{9}\)
Dấu "." là dấu nhân nhé
\(\frac{1}{2.4}+\frac{1}{4.6}+\frac{1}{6.8}+...+\frac{1}{2016.2018}\)
\(=\frac{1}{2}.\left(\frac{2}{2.4}+\frac{2}{4.6}+...+\frac{2}{2016.2018}\right)\)
\(=\frac{1}{2}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{2016}-\frac{1}{2018}\right)\)
\(=\frac{1}{2}.\left(1-\frac{1}{2018}\right)\)
\(=\frac{1}{2}.\frac{2017}{2018}\)
\(=\frac{2017}{4036}\)
\(x.\frac{2}{7}.\frac{3}{4}=\frac{5}{21}\)
\(\frac{3}{14}x=\frac{5}{21}\)
\(x=\frac{5}{21}:\frac{3}{14}\)
\(x=\frac{5}{21}.\frac{14}{3}\)
\(x=\frac{10}{9}\)
Vậy \(x=\frac{10}{9}\)
những câu bài 2 này là tìm x bình thường