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A = 1 2.3 + 2 3.4 + 1 4.5 + 1 5.6 + 1 6.7 + 1 7.8
1 2 − 1 3 + 1 3 − 1 4 + 1 4 − 1 5 + 1 5 − 1 6 + 1 6 − 1 7 + 1 7 − 1 8 = 3 8
![](https://rs.olm.vn/images/avt/0.png?1311)
S = 1 2 + 1 6 + 1 12 + 1 20 + 1 30 + 1 42 = 1 1.2 + 1 2.3 + 1 3.4 + 1 4.5 + 1 5.6 + 1 6.7 = 1 − 1 2 + 1 2 − 1 3 + 1 3 − 1 4 + 1 4 − 1 5 + 1 5 − 1 6 + 1 6 − 1 7 = 1 − 1 7 < 1
So sánh: S < 1.
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B = 1 2 + 1 6 + 1 12 + 1 20 + 1 30 + 1 42 + 1 56 + 1 72 + 1 90
B = 1 1.2 + 1 2.3 + 1 3.4 + 1 4.5 + 1 5.6 + 1 6.7 + 1 7.8 + 1 8.9 + 1 9.10
= 1 − 1 2 + 1 2 − 1 3 + 1 3 − 1 4 + 1 4 − 1 5 + ... + 1 9 − 1 10
= 1 + − 1 2 + 1 2 + − 1 3 + 1 3 + − 1 4 + 1 4 + − 1 5 + 1 5 + ... + − 1 9 + 1 9 − 1 10
= 1 − 1 10 = 9 10
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a) (234 + 576 + 552) : 3 = 454
b) (145 + 130 + 153 + 142 + 150) : 5 = 144
Chúc em học tốt
![](https://rs.olm.vn/images/avt/0.png?1311)
a) 142 x 12 + 142 x 18
= 142 x ( 18 + 12 )
= 142 x 30
= 4260
b) 49 . 365 - 39 . 365
= 365 . ( 49 - 39)
= 365 . 10
= 3650
c) 4 . 18 . 25
= 18 . 4 . 25
= 18 . 100
= 1800
Trả lời :
\(142-4\times x=130\)
\(4x=142-130\)
\(4x=12\)
\(x=12\div4\)
\(x=3\)
142 - 4 x X =130
4 x X = 142 - 130
4 x X = 12
X = 12 : 4
X = 3
Vậy X = 3
HOK T ~