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8 tháng 11 2017
3S= 3/2.5+3/5.8+....+3/17.20
= (5-2)/2.5+(8-5)/5.8+....+(20-17)/17.20
= 1/2 - 1/5 + 1/5 - 1/8 + ...... + 1/17 - 1/20 = 1/2 - 1/20 = 9/20
=> S = 3/20
8 tháng 11 2017
\(S=\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{17.20}\)
\(S=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{17}-\frac{1}{20}\right)\)
\(S=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{20}\right)\)
\(S=\frac{1}{3}.\frac{9}{20}\)
\(S=\frac{3}{20}\)
5 tháng 11 2017
In my school, there are many teachers.I love all the teachers who taught me but the one i loved the most was teaching English while studying in elemantary school. her name is Tham , Miss Tham is about 29 years old and has about 1m64cm ,she has long hair on her back, long black hair and smooth. Facial oval eyes with large double eyelid clear. While teaching you if you have a fever, she will take you to the medical center immediately. She is sociable and happy with everyone whether her friends and students . She loves us all and we love her very much.
DD
5 tháng 11 2017
Everyone sat up expectantly. It was a maths period and all of us waited eagerly for our maths teacher, Mrs Huang, to come into the class. She is very popular among us. In fact, she is my favorite teacher.
Mrs Huang has short curly hair and wears thick glasses, which makes her look stern. However, she is actually very approachable and is always ready to lend an ear to our problems. She is motherly and cares very much for us. I remember once when I fell and hurt my knee. Mrs Huang helped me to stop the bleeding and took me to the clinic. I was very grateful to her.
I used to dis maths when I was younger because I found it far too complicated. Mrs Huang always makes maths lessons interesting and easy to understand. She is very patient in explaining maths problems to us and always makes sure everyone understands. Under her guidance, my maths has improved by leaps and bounds.
I have grown to love maths and I always score high marks in my maths tests now. It is all because of my favorite teacher.
21 tháng 6 2016
\(S=\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{17.20}\)
\(S=3.\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{17}-\frac{1}{20}\right)\)
\(S=3.\left(\frac{1}{2}-\frac{1}{20}\right)\)
\(S=3.\frac{9}{20}=\frac{27}{20}\)
21 tháng 6 2016
\(a.x+9=2\frac{1}{3}\)
\(x=2\frac{1}{3}-9\)
\(x=-\frac{20}{3}\)
\(b.\frac{-5}{2}.x=\frac{1}{10}\)
\(x=\frac{1}{10}:\frac{-5}{2}\)
\(x=-\frac{1}{25}\)
21 tháng 6 2016
a)
<=> x=\(\frac{7}{3}-9=-\frac{20}{3}\)
b)
<=> x=\(\frac{1}{10}:\frac{-5}{2}=-\frac{2}{25}\)
21 tháng 6 2016
\(a.\frac{2}{3}+\frac{5}{7}=\frac{14}{21}+\frac{15}{21}=\frac{19}{21}\)
\(b.\frac{3}{2}.\frac{4}{5}-\frac{1}{35}=\frac{6}{5}-\frac{1}{35}=\frac{42}{35}-\frac{1}{35}=\frac{41}{35}\)
\(c.\left(\frac{20}{3}-\frac{22}{5}\right):\frac{1}{15}=\left(\frac{100}{15}-\frac{66}{15}\right):\frac{1}{15}=\frac{34}{15}:\frac{1}{15}=\frac{34}{15}x15=34\)
21 tháng 6 2016
a) \(\frac{2}{3}+\frac{5}{7}=\frac{14}{21}+\frac{15}{21}=\frac{14+15}{21}=\frac{29}{21}\)
b)\(\frac{3}{2}.\frac{4}{5}-\frac{13}{5}=\frac{3.4}{2.5}-\frac{13}{5}=\frac{12}{10}-\frac{13}{5}=\frac{6}{5}-\frac{13}{5}=-\frac{7}{5}\)
c)\(\left(\frac{20}{3}-\frac{22}{5}\right):\frac{1}{15}=\left(\frac{100}{15}-\frac{66}{15}\right):\frac{1}{15}=\frac{44}{15}.15=44\)
9 tháng 12 2016
40 hoặc 96
Cách lý giải cho đáp án 40: Cộng đáp án của dòng trên vào những con số của dòng dưới. Cụ thể:
1 + 4 = 5
5 + 2 + 5 = 12
12 + 3 + 6 = 21
21 + 8 + 11 = 40
Cách lý giải cho đáp án 96: Nhân số thứ nhất với số thứ hai trong mỗi dòng, và cộng với số thứ nhất. Cụ thể:
1 + (1 x 4) = 5
2 + (2 x 5) = 12
3 + (3 x 6) = 21
8 + (8 x 11) = 96
hay đấy tớ cho 10 điểm nha