English has been one of the most widely spoken languages worldwide for a really long time. If you need to study abroad, you must know how to speak English fluently, especially if you don't know how to speak the country's native language. Because of that, English has become really important to everyone. And for that, I decided to create an app called "EngEdu". EngEdu is an app that will help you learn English online anywhere. It is pretty simple to use this app. To begin with, you must make an account and set up your profile. You can customize your profile as you like. After setting up your profile, you may start learning. This app covers all of the basic skills, including reading, writing, listening, and speaking, and also provides you with vocabulary and grammar lessons. You can also connect with other learners with the app's chatting feature. Besides all of the basic learning features, EngEdu also has multiple more fun features, such as competition, practice, assessment, etc. To conclude, I assure you that this app can help you reach higher levels in your English and will be of great help in your career.

Mẫu số chung: 70

\(\dfrac{3}{5}=\dfrac{3\times14}{5\times14}=\dfrac{42}{70}\)

\(\dfrac{4}{7}=\dfrac{4\times10}{7\times10}=\dfrac{40}{70}\)

\(\dfrac{5}{10}=\dfrac{5\times7}{10\times7}=\dfrac{35}{70}\)

\(\dfrac{1}{10}+\dfrac{1}{15}+\dfrac{1}{21}+...+\dfrac{1}{105}\)

\(=2\times\left(\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+...+\dfrac{1}{210}\right)\)

\(=2\times\left(\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+...+\dfrac{1}{14\times15}\right)\)

\(=2\times\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+...+\dfrac{1}{14}-\dfrac{1}{15}\right)\)

\(=2\times\left(\dfrac{1}{4}-\dfrac{1}{15}\right)\)

\(=2\times\left(\dfrac{15}{60}-\dfrac{4}{60}\right)\)

\(=2\times\dfrac{11}{60}\)

\(=\dfrac{22}{60}=\dfrac{11}{30}\)

a. Nguời ấy được lãi số tiền là:
42000 x 25% = 10500(đồng)

b. Người ấy bán rau được số tiền là:
42000 + 10500 = 52500(đồng)

c. Người ấy được lãi số phần trăm so với giá bán là:
10500 : 52500 x 100% = 20%

87 x 25 + 114 x 25 - 25

= 87 x 25 + 114 x 25 - 1 x  25

= (87 + 114 - 1) x 25

= 200 x 25

= 5000