Question

[VangThăng Long PKD siêu thị]

tính nhanh. 1/3×4+2/4×6+3/6×9+4/9×13+5/13×18.

SOS CÚ TUI VS ;-;

Answer 1

[HT.Phong (9A5)]

\(\dfrac{1}{3\times4}+\dfrac{2}{4\times6}+\dfrac{3}{6\times9}+\dfrac{4}{9\times13}+\dfrac{5}{13\times18}\)

\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{18}\)

\(=\dfrac{1}{3}-\dfrac{1}{18}\)

\(=\dfrac{5}{18}\)