Ta có: \(\frac{1}{5}+\frac{1}{6}+...+\frac{1}{17}\)

\(=\left(\frac{1}{5}+\frac{1}{6}+...+\frac{1}{10}\right)+\left(\frac{1}{11}+...+\frac{1}{17}\right)\)

\(< \frac{1}{5}.6+\frac{1}{11}.6=\frac{5}{6}+\frac{6}{11}=\frac{101}{55}< 2\)

một bạn sẽ ăn chung hoặc nhịn

                                         My best friend name's Như.

                             She has long black hair and big eyes.

                             She is light.She play badminton with me.

                             She swimming.And she eat noodles

                             and drink orange juice.

                             I love my best friend

My best friend in high school is Lan. Lan sits next to me in class, eats with me at break time, and hangs out with me after school. She has long and beautiful hair; my mother always says that I need to pay more attention to my hair so it can be pretty as hers. I am good at English while Lan is excellent in Math, therefore we can help each other every time one of us has trouble in understanding those subjects. Besides our study, Lan is always by my side whenever I feel upset. I tell her everything, and only her can know my true feelings. We have been best friends since our young age, and I will keep this friendship forever.

Tham khảo nha!

My best friend in high school is Lan. Lan sits next to me in class, eats with me at break time, and hangs out with me after school. She has long and beautiful hair; my mother always says that I need to pay more attention to my hair so it can be pretty as hers. I am good at English while Lan is excellent in Math, therefore we can help each other every time one of us has trouble in understanding those subjects. Besides our study, Lan is always by my side whenever I feel upset. I tell her everything, and only her can know my true feelings. We have been best friends since our young age, and I will keep this friendship forever.

Hc tốt #

X = -168

k mk nha

nhanh lên

sai rùi hotboylanhlung ơi

co 1 so 1

Ai tích mk mk sẽ tích lại

dãy số 1; 2; 3; 4 có 1 chữ số 1

trong dãy số đó có 1 chữ số 1

0 chứ mấy

Ta thấy trong dãy số có số 5 và 6

5 x 6 = 30

Có tận cùng là 0 

Ta thấy số nào có chữ số 0 ở tận cùng thì số đó cũng có số 0 ở tận cùng

Vậy chữ số tận cùng là 0

thank you very much

cảm ơn bạn rất nhiều

\(\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{49.50.51}\)

\(=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{49.50}-\frac{1}{50.51}\right)\)

\(=\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{50.51}\right)\)

\(=\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{2550}\right)\)

\(=\frac{1}{2}.\left(\frac{1275}{2550}-\frac{1}{2550}\right)\)

\(=\frac{1}{2}.\frac{1274}{2550}\)

\(=\frac{637}{2550}\)

Lưu ý : Dấu \("."\)là dấu \("\)x  \("\)

( dấu nhân ) 

Chúc bạn học giỏi !!! 

Công thức : 

\(\frac{1}{2}.\left(\frac{1}{1.2}-\frac{1}{2.3}\right)=\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{6}\right)=\frac{1}{2}.\frac{2}{6}=\frac{1}{6}=\frac{1}{1.2.3}\)

VD ( dễ hiểu ) 

\(\frac{637}{2550}\)

Chúc bạn học tốt

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