Question

[Bùi Duy Hải]

3/4x7+3/7x10+3/10x13+...+6/87/90

Answer 1

[HT.Phong (9A5)]

\(\dfrac{3}{4\cdot7}+\dfrac{3}{7\cdot10}+\dfrac{3}{10\cdot13}+...+\dfrac{3}{87\cdot90}\)

\(=\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{10}+...+\dfrac{1}{87}-\dfrac{1}{90}\)

\(=\dfrac{1}{4}+\left(\dfrac{1}{7}-\dfrac{1}{7}\right)+\left(\dfrac{1}{10}-\dfrac{1}{10}\right)+...+\left(\dfrac{1}{87}-\dfrac{1}{87}\right)-\dfrac{1}{90}\)

\(=\dfrac{1}{4}-\dfrac{1}{90}\)

\(=\dfrac{45}{180}-\dfrac{2}{180}\)

\(=\dfrac{43}{180}\)

Answer 2

[Kiều Vũ Linh]

Em xem lại phân số cuối nhé, em viết sai rồi