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![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\frac{10}{5.7}+\frac{10}{7.9}+\frac{10}{9.11}+...+\frac{10}{33.35}\)
\(\Rightarrow A=10.\left(\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+...+\frac{1}{33.35}\right)\)
Đặt \(B=\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}+...+\frac{1}{33.35}\)
\(\Rightarrow2B=\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+...+\frac{2}{33.35}\)
\(\Rightarrow2B=\frac{1}{5}-\frac{1}{35}=\frac{6}{35}\)
\(\Rightarrow B=\frac{3}{35}\)
\(\Rightarrow A=10.\frac{3}{35}=\frac{30}{35}=\frac{6}{7}\)
_Học tốt_
![](https://rs.olm.vn/images/avt/0.png?1311)
Tính:
a, 9-9x9+9-9
= 9 - 81 + 9 - 9
= - 72 + 0
= -72
b, 10-10x10x10
= 10 - 10 x 100
= 10 - 1000
= -990
c, 11+11:11+11
= 11 + 1 + 11
= 12 + 11
= 23
d, 12x12:12:12
= 144 : 1
= 144
![](https://rs.olm.vn/images/avt/0.png?1311)
9/10 của A là 9.==> A = 9 : 9/10 = 10
Nếu A x 8 thì A = 10 x 8 = 80
Đ/S : 80
![](https://rs.olm.vn/images/avt/0.png?1311)
\(1\frac{1}{10}+2\frac{2}{10}+3\frac{3}{10}+4\frac{4}{10}+...+9\frac{9}{10}\)
\(=\frac{11}{10}+\frac{22}{10}+\frac{33}{10}+\frac{44}{10}+...+\frac{99}{10}\)
\(=1,1+2,2+3,3+4,4+...+9,9\)
\(=\frac{\left(9,9+1,1\right).9}{2}=49.5\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a)\(3\frac{4}{10}>2\frac{9}{10}\)
b)\(3\frac{4}{10}< 3\frac{9}{10}\)
c)\(5\frac{1}{10}>2\frac{9}{10}\)
d)\(3\frac{4}{10}=3\frac{2}{5}\)
Hok tốt!~
![](https://rs.olm.vn/images/avt/0.png?1311)
a). 3 9/10 > 2 9/10
b).3 4/10 < 3 9/10
c).5 1/10 > 2 9/10
d).3 4/10 = 3 2/5
![](https://rs.olm.vn/images/avt/0.png?1311)
E = 1x2x3 + 2x3x4 + 3x4x5 + ... + 9x10x11
E x 4 = 1x2x3x4 + 2x3x4x4 + 3x4x5x4 + ... + 9x10x11x4
E x 4 = 1x2x3x4 + 2x3x4x(5-1) + 3x4x5x(6-2) + ... + 9x10x11x(12-8)
E x 4 = 1x2x3x4 + 2x3x4x5 - 1x2x3x4 + 3x4x5x6 - 2x3x4x5 + ... + 9x10x11x12 - 8x9x10x11.
E x 4 = 9x10x11x12
E = 9x10x11x12 : 4
E = 2970
10-9=1
a=1
a + 9 = 10
a = 10 - 9
a = 1
Vậy a = 1
Ủng hộ nhé ! Bấm Đúng nếu thấy mình trả lời đúng !