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Mấy câu khác bạn tự tính nhé, dễ thôi. Câu 2, câu 3 giống dạng nhau. Dấu . có nghĩa là dấu nhân nhé.
Câu 2:
\(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(=\frac{1}{2}-\frac{1}{8}\)
\(=\frac{3}{8}\)
Câu 4 b) bạn quy đồng 2 phân số cho cùng mẫu rồi tìm thôi.
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Bài 7:
Số phần kẹo Hùng đã cho Hà và Hồng là:
\(\dfrac{2}{7}+\dfrac{1}{7}=\dfrac{3}{7}\left(phần\right)\)
Hùng còn lại số phần của gói kẹo là:
\(\dfrac{6}{7}-\dfrac{3}{7}=\dfrac{3}{7}\left(phần\right)\)
1:
2 3/4
5 6/5
3 3/9
7 6/8
2:
1/3 + 2/3 + (3/4 + 1/4) = 2
=2
= 4 5/10
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1/10+2/9+3/8+4/7+5/6+1/6+3/7+5/8+7/9+9/10
=(1/10+9/10)+(2/9+7/9)+(3/8+5/8)+(4/7+3/7)+(5/6+1/6)
=1+1+1+1+1
=5
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A = \(\frac{1}{2.3}\)+ \(\frac{1}{3.4}\) + \(\frac{1}{4.5}\)+...+\(\frac{1}{9.10}\)
= (\(\frac{1}{6}\) + \(\frac{1}{12}\)) + ( \(\frac{1}{20}\)+\(\frac{1}{30}\) ) + ( \(\frac{1}{42}\)+\(\frac{1}{56}\) ) + ( \(\frac{1}{72}\)+ \(\frac{1}{90}\) )
= \(\frac{1}{4}\) + \(\frac{1}{12}\) + \(\frac{1}{24}\) + \(\frac{1}{40}\)
= (\(\frac{1}{4}\)+ \(\frac{1}{12}\) ) + ( \(\frac{1}{24}\)+ \(\frac{1}{40}\) )
= \(\frac{1}{3}\) + \(\frac{1}{15}\)
= \(\frac{2}{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có
\(\frac{1}{10}+\frac{2}{10}+\frac{3}{10}+\frac{4}{10}+\frac{5}{10}+\frac{6}{10}+\frac{7}{10}+\frac{8}{10}+\frac{9}{10}\)
\(=\frac{1+2+3+4+5+6+7+8+9}{10}\)
\(=\frac{\left(1+9\right)+\left(2+8\right)+\left(3+7\right)+\left(4+6\right)+5}{10}=\frac{10.4+5}{10}=\frac{45}{10}=\frac{9}{2}\)
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\(\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{7\times8}+....+\dfrac{1}{9\times10}\)
\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+.....+\dfrac{1}{9}-\dfrac{1}{10}\)
\(=\dfrac{1}{3}-\dfrac{1}{10}\)
\(=\dfrac{7}{30}\)
Sửa đề:
\(\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+\dfrac{1}{8\times9}+\dfrac{1}{9\times10}\)
\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\)
\(=\dfrac{1}{3}-\dfrac{1}{10}\)
\(=\dfrac{10}{30}-\dfrac{3}{30}\)
\(=\dfrac{7}{30}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
b)1/2*2/3*3/4*4/5*5/6*6/7*7/8*8/9*9/10
=1x2x3x4x5x6x7x8x9
2x3x4x5x6x7x8x9x10
= 1
10
\(=\dfrac{720}{7}\cdot8-9+10\cdot15\)
=5760/7+141
=6747/7