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\(E=\dfrac{3}{1x5}+\dfrac{3}{5x9}+...+\dfrac{3}{121x125}\)
\(\dfrac{4}{3}xE=1-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{9}+...+\dfrac{1}{121}-\dfrac{1}{125}\)
\(\dfrac{4}{3}xE=1-\dfrac{1}{125}\)
\(E=\dfrac{124}{125}x\dfrac{3}{4}=\dfrac{93}{125}\)
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Lời giải:
$12A=1.5.12+5.9.(13-1)+9.13(17-5)+13.17(21-9)+....+77.81(85-73)+81.85(89-77)$
$=60+(5.9.13+9.13.17+13.17.21+...+77.81.85+81.85.89)-(1.5.9+5.9.13+9.13.17+...+73.77.81+77.81.85)$
$=60+81.85.89 - 1.5.9=612780$
A = 1.5 + 5.9 + 9.13 + ... + 81.85
A = \(\dfrac{12}{12}\)(1.5 + 5.9 + 9.13 + 81.85)
A = \(\dfrac{1}{12}\).(1.5.12 + 5.9.12.+ 9.13.12 + ...+ 81.85.12]
A = \(\dfrac{1}{12}\).[1.5.(9 + 3) + 5.9.(13 - 1) + 9.13.(17 - 5) +...+ 81.85.(89 - 77)]
A = \(\dfrac{1}{12}\).[1.5.9 + 1.3.5 + 5.9.13 - 5.9.1 + 9.13.17 - 9.13.5 + ...+ 81.85.89 - 81.85.77]
A = \(\dfrac{1}{12}\).[1.3.5 + 81.85.89]
A = \(\dfrac{1}{12}\).[15 + 612765]
A = \(\dfrac{1}{12}\).612780
A = 51065
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1 x 2 - 10 x 5 - 5 x 9
= 2 - 50 - 45
= 2 -( 50 + 45)
= 2 - 95
= -93
\(1\times2-10\times5-5\times9\\ =2-\left(10-9\right)\times5\\ =2-1\times5\\ =2-5\\ =-3\)
Xem lại lớp.
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làm bừa thui,ai tích mình mình tích lại
Số số hạng là :
Có số cặp là :
50 : 2 = 25 ( cặp )
Mỗi cặp có giá trị là :
99 - 97 = 2
Tổng dãy trên là :
25 x 2 = 50
Đáp số : 50
A = 1.5 + 5.9 + 9.13 + ...+ 41.45
A = \(\dfrac{12}{12}\).(1.5 + 5.9 + 9.13 +...+ 41.45)
A = \(\dfrac{1}{12}\).(1.5.12 + 5.9.12 + 9.13.12 + ... + 41.45.12)
A = \(\dfrac{1}{12}\).[1.5.(9 + 3) + 5.9.(13 - 1) + 9.13.(17 - 5) + ... + 41.45.(49 - 37)]
A = \(\dfrac{1}{12}\)[1.5.9 + 1.5.3 + 5.9.13 - 1.5.9 + 9.13.17 - 5.9.13 +...+41.45.49 - 41.45.37]
A =\(\dfrac{1}{12}\).[1.5.3 + 41.45.49]
A = \(\dfrac{1}{12}\).[15 + 90405]
A = \(\dfrac{1}{12}\).90420
A = 7535
.
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\(\frac{2}{5.9}+\frac{2}{9.13}+\frac{2}{13.27}+...+\frac{2}{55.59}\)
\(=\frac{1}{2}.\left(\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{27}+...+\frac{1}{55}-\frac{1}{59}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{5}-\frac{1}{59}\right)\)
\(=\frac{1}{2}.\frac{54}{295}\)
\(=\frac{27}{259}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
108 nha Nguyễn Việt hoàng
3+5x9+5x12
=3+45+60
=48+60
=108