Chúc mừng các anh, chị CTV nha!

Chúc mừng mn.

\(\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}\\ =\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}\\ =\dfrac{1}{2}-\dfrac{1}{5}\\ =\dfrac{3}{10}\)

How much is that sweater cost?

S - Subject : Chủ ngữ

V - Verb : động từ

\(\dfrac{2}{1\cdot3}+\dfrac{2}{3\cdot5}+...+\dfrac{2}{43\cdot45}\\ =\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+...+\dfrac{1}{43}-\dfrac{1}{45}\\ =\dfrac{1}{1}-\dfrac{1}{45}\\ =\dfrac{44}{45}\)

How many students does you school have?

Sửa lại bài của mình nhé:

\(A=2\times\left(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}\right)\\ A=2\times\left(\dfrac{1}{5}-\dfrac{1}{15}\right)\\ A=2\times\dfrac{2}{15}\\ A=\dfrac{4}{15}\)

Sửa đề: \(\dfrac{4}{5\times7}+\dfrac{4}{7\times9}+\dfrac{4}{9\times11}+\dfrac{4}{11\times13}+\dfrac{4}{13\times15}\)

Gọi A = \(\dfrac{4}{5\times7}+\dfrac{4}{7\times9}+\dfrac{4}{9\times11}+\dfrac{4}{11\times13}+\dfrac{4}{13\times15}\)

\(A=\dfrac{4}{5\times7}+\dfrac{4}{7\times9}+\dfrac{4}{9\times11}+\dfrac{4}{11\times13}+\dfrac{4}{13\times15}\\ A=2\times\left(\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+\dfrac{2}{9\times11}+\dfrac{2}{11\times13}+\dfrac{2}{13\times15}\right)\\ A=2\times\left(\dfrac{1}{3}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}\right)\\ A=2\times\left(\dfrac{1}{3}-\dfrac{1}{15}\right)\\ A=2\times\dfrac{4}{15}\\ A=\dfrac{8}{15}\)

Vậy \(A=\dfrac{8}{15}\)

Ta có:

\(\dfrac{12}{13}=\dfrac{12}{13};\dfrac{1212}{1313}=\dfrac{1212:101}{1313:101}=\dfrac{12}{13}\)

Vì \(\dfrac{12}{13}=\dfrac{12}{13}\) ⇒ \(\dfrac{12}{13}=\dfrac{1212}{1313}\)