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![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{5}{18.21}+\frac{5}{21.24}+\frac{5}{24.27}+...+\frac{5}{123.126}\)
\(=\frac{5}{3}\left(\frac{3}{18.21}+\frac{3}{21.24}+\frac{3}{24.27}+...+\frac{3}{123.126}\right)\)
\(=\frac{5}{3}\left(\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+\frac{1}{24}-\frac{1}{27}+...+\frac{1}{123}-\frac{1}{126}\right)\)
\(=\frac{5}{3}\left(\frac{1}{18}-\frac{1}{126}\right)\)
\(=\frac{5}{3}.\frac{1}{21}\)
\(=\frac{5}{63}\)
Study well ! >_<
![](https://rs.olm.vn/images/avt/0.png?1311)
bạn sai ở đoạn cuối
A=\(\frac{3\cdot3}{15\cdot18}+\frac{3\cdot3}{18\cdot21}+...+\frac{3\cdot3}{96\cdot99}=3\cdot\left(\frac{3}{15\cdot18}+\frac{3}{18\cdot21}+...+\frac{3}{96\cdot99}\right)=3\cdot\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{96}-\frac{1}{99}\right)=3\cdot\left(\frac{1}{15}-\frac{1}{99}\right)=3\cdot\frac{28}{495}=\frac{28}{165}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
B8 = 6/15.18 + 6/18.21 + 6/21.24 + ... + 6/87.90
B8 = 2.(3/15.18 + 3/18.21 + 3/21.24 + ... + 3/87 .90)
B8 = 2.(1/15 - 1/18 + 1/18 -1/21 + 1/21 - 1/24 + ... + 1/87 - 1/90)
B8 = 2.(1/15 - 1/90)
B8 = 2.(6/90 - 1/90)
B8 = 2. 1/18
B8 = 1/9
B = 1/9 : 8
B = 1/72
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Ta có :
\(\frac{3}{18x21}+\frac{3}{21x24}+...+\frac{3}{123x126}\)
\(=\frac{3}{18}-\frac{3}{21}+\frac{3}{21}-\frac{3}{24}+...+\frac{3}{123}-\frac{3}{126}\)
\(=\frac{3}{18}-\frac{3}{126}\)
\(=\frac{1}{7}\)
~ Thiên Mã ~
3/(18x21) +3/(21x24) + 3/(24 x 27) + .... +3/(123x 126)
=1/18-1/21+1/21-1/24+...+1/123-1/126
=1/18-1/126
=7/126-1/126
=6/126=1/21
![](https://rs.olm.vn/images/avt/0.png?1311)
D=6/15.18+6/18.21+...+6/89.92
D=6(1/15.18+1/18.21+...+1/89.92)
a) 3D=6(1/15-1/18+1/18-1/21+...+1/89-1/92)
3D=6(1/15-1/92)
3D=6.(77/1380)
3D=77/230
D=77/690
b) F=1/25.27+1/27.29+...+1/73.75
2F=2/25.27+2/27.29+..+2/73.75
2F=1/25-1/27+1/27-1/29+...+1/73-1/75
2F=1/25-1/75
2F=2/75
F=1/75
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
A=24x27-23/24+23x27
A=(23+1)x27-23/24+23x27
A=23x27+27-23/24+23x27
A=23x27+4/24+23x27
A=4/24
A=1/6
A=\(\frac{23x27+27-23}{23x27+24}\)
A=\(\frac{23x27+4}{23x27+24}\)
A=\(\frac{625}{645}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
A.3=5.3/18.21+5.3/21.24+5.3/24.27+...+5.3/123.126
A.3=5/18-5/21 + 5/21-5/24 + 5/24-5/27 +...+ 5/123-5/126
A.3=5/18-5/126
A.3= 5/21
A =5/21:3
A = 5/63
Đây là cách của mik nếu sai thì thôi nhé
Kết quả đúng rồi đấy !