Question

[van tien dung]

tính nhanh

A=2/1.3+2/3.5+2/5.7+...+2/49.51

B=2/5+2/7-1/6-1/8+11/35-34/48+11/12

Answer 1

[Đỗ Thị Minh Ánh]

A=\(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{49.51}\)

=\(\frac{2}{1}-\frac{2}{3}+\frac{2}{3}-\frac{2}{5}+\frac{2}{5}-\frac{2}{7}+...+\frac{2}{49}-\frac{2}{51}\)

= \(2.(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{49}-\frac{1}{51})\)

=2.\((1-\frac{1}{51})\)

=\(2.\frac{50}{51}\)

=\(\frac{100}{51}\)