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Mk ko ghi lại đề nhé
1 = \(350-4.19+4.7\)
\(=350-4.\left(19+7\right)\)
\(=350+4.26\)
\(=350-104=246\)
2 Câu này mình vẫn chưa hiểu là bạn ghi 27 3/5 tức là 27.3/5 hay 27\(\dfrac{3}{5}\)nên mk bỏ qua nhé
3 Cái đoạn 1 1/3 y chang câu 2
4 \(=\dfrac{2}{3}+\dfrac{1}{3}.\dfrac{7}{18}.\dfrac{12}{7}\)
\(=\dfrac{2}{3}+\dfrac{7}{54}.\dfrac{12}{7}\)
\(=\dfrac{2}{3}+\dfrac{2}{9}=\dfrac{8}{9}\)
5 \(=2^2+10^2-\left[9.\left(112:8\right)-11\right].1\)
\(=4+100-\left(9.14-11\right)\)
\(=104-115\)\(=-11\)
6 Đoạn 6 3/5 y chang 2,3
7 \(=\left(\dfrac{2}{3}-1\right)+\left(\dfrac{2}{7}-\dfrac{3}{7}\right)-\left(\dfrac{1}{14}-\dfrac{3}{28}\right)\)
\(=\dfrac{-1}{3}+\dfrac{-1}{7}-\dfrac{-1}{28}\)
\(=\dfrac{-196}{588}+\dfrac{-84}{588}-\dfrac{-21}{588}\)\(=\dfrac{259}{588}\)
Làm tạm tạm thôi xl vì có 3 câu ko hiểu
mik cách ra nghĩa là ghi 27, 3 phần 5 (mấy câu khác cx zị)hog phải nhân nha
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Ta có:\(\frac{1}{2.9}=\frac{1}{2}-\frac{1}{9}\)
\(\frac{1}{9.7}=\frac{1}{9}-\frac{1}{7}\)
\(⋮\)
\(\frac{1}{252.504}=\frac{1}{252}-\frac{1}{504}\)
\(A=\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{7}+\frac{1}{7}-...............+\frac{1}{252}-\frac{1}{504}\)
\(A=\frac{1}{2}-\frac{1}{504}\)
\(A=\frac{251}{504}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Đặt \(A=\frac{1}{2.9}+\frac{1}{9.7}+\frac{1}{7.19}+...+\frac{1}{252.509}\)
\(\Leftrightarrow A=\frac{2}{5}.\left(\frac{5}{4.9}+\frac{5}{9.14}+\frac{5}{14.19}+...+\frac{5}{504.509}\right)\)
\(\Leftrightarrow A=\frac{2}{5}.\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{19}+...+\frac{1}{504}-\frac{1}{509}\right)\)
\(\Leftrightarrow A=\frac{2}{5}.\left(\frac{1}{4}-\frac{1}{509}\right)\)
\(\Leftrightarrow A=\frac{2}{5}.\frac{505}{2036}\)
\(\Leftrightarrow A=\frac{101}{1018}\)
~ Hok tốt ~
#)Giải :
\(A=\frac{1}{2.9}+\frac{1}{9.7}+\frac{1}{7.19}+...+\frac{1}{252.509}\)
\(A=\frac{2}{5}\left(\frac{5}{4.9}+\frac{5}{9.14}+\frac{5}{14.19}+...+\frac{5}{504.509}\right)\)
\(A=\frac{2}{5}\left(\frac{1}{4}-\frac{1}{9}+\frac{1}{9}-\frac{1}{14}+\frac{1}{14}-\frac{1}{17}+...+\frac{1}{504}-\frac{1}{509}\right)\)
\(A=\frac{2}{5}\left(\frac{1}{4}-\frac{1}{509}\right)\)
\(A=\frac{2}{5}\times\frac{505}{2036}\)
\(A=\frac{101}{1018}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{1}{7}\left[\frac{1}{2}-\frac{1}{9}+...+\frac{1}{252}-\frac{1}{509}\right]\)
\(A=\frac{1}{7}.\left[\frac{1}{2}-\frac{1}{509}\right]\)
\(A=\frac{1}{7}.\left[\frac{507}{1018}\right]=\frac{507}{7126}\)
mk nghĩ là vậy đó, ủng hộ mk nha
\(19\times5+7\times19+12-18\times12\)
\(=19\left(5+7\right)+12\left(1-18\right)\)
\(=19\times12+12\times\left(-17\right)\)
\(=12\left[19+\left(-17\right)\right]\)
\(=12\times2=24\)
19.5+7.19+12-18.12=19(5+7) +12(1-18)
=19.12+12(-17)
=12(19-17)
=12.2=24