giúp mình câu này nhé.Cảm ơn
tính:I=1/2x15+1/15x3+1/3x21+...+6/87x90
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Những câu hỏi liên quan
23 tháng 3 2022
Gọi biểu thức trên là A, ta có:
\(A=\frac{1}{2\cdot15}+\frac{1}{15\cdot3}+\frac{1}{3\cdot21}+\frac{1}{21\cdot4}+...+\frac{1}{87\cdot90}\)
\(13A=\frac{13}{2\cdot15}+\frac{13}{15\cdot3}+\frac{13}{3\cdot21}+\frac{13}{21\cdot4}+...+\frac{13}{87\cdot90}\)
\(13A=\frac{1}{2}-\frac{1}{15}+\frac{1}{15}-\frac{1}{3}+\frac{1}{3}-\frac{1}{21}+\frac{1}{21}-\frac{1}{4}+...+\frac{1}{87}-\frac{1}{90}\)
\(13A=\frac{1}{2}-\frac{1}{90}\)
\(13A=\frac{22}{45}\)
\(A=\frac{22}{45\text{x}13}=\frac{22}{585}\)
9 tháng 7 2015
\(\frac{A}{6}=\frac{1}{12\times15}+\frac{1}{15\times18}+\frac{1}{18\times21}+...+\frac{1}{87\times90}\)
\(\frac{A}{6}=\frac{1}{3}\times\left(\frac{3}{12\times15}+\frac{3}{15\times18}+\frac{3}{18\times21}+...+\frac{3}{87\times90}\right)\)
\(\frac{A}{6}=\frac{1}{3}\times\left(\frac{1}{12}-\frac{1}{15}+\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{87}-\frac{1}{90}\right)\)
\(\frac{A}{6}=\frac{1}{3}\times\left(\frac{1}{12}-\frac{1}{90}\right)=\frac{1}{3}\times\frac{13}{180}=\frac{13}{540}\)=> A = \(\frac{13}{90}\)
LQ
0
NT
Nguyễn Thị Thương Hoài
Giáo viên
VIP
13 tháng 7 2023
D = \(\dfrac{1}{1.4}\) + \(\dfrac{1}{4.7}\) + \(\dfrac{1}{7.10}\)+...+ \(\dfrac{1}{91.94}\)
D = \(\dfrac{1}{1}\) - \(\dfrac{1}{4}\) + \(\dfrac{1}{4}\) - \(\dfrac{1}{7}\) + \(\dfrac{1}{7}\) - \(\dfrac{1}{10}\)+...+ \(\dfrac{1}{91}\) - \(\dfrac{1}{94}\)
D = \(\dfrac{1}{1}\) - \(\dfrac{1}{94}\)
D = \(\dfrac{93}{94}\)
HA
1
HL
13 tháng 8 2016
câu cảm thán với What:
1.What a lazy student!
2. What an interesting novel!
3.What tight shoes are!
4.What beautiful flowers are!
5. What beautiful weather!
6.What a good picture they saw!
7.What a cute baby!
8.What an interesting film!
9.What large apartments!
10.What nice weather!
11.What a big challenge you have!
12.What a hot day it is today!
13.What an amazing player John is!
14.What beautiful flowers he sent!
15.What lovely weather it is!
16.What a great lesson we have!
17.What a surprise!
18.What a good idea!
19.What a quiet family picture!
20.What awful weather!
Câu cảm thán với How
1.How cold it is!
2.How interesting this gift is!
3.How good it smells!
4. How well she sings!
5.How beautiful she is!
6.How carefully he drives!
7.How kind you are!
8.How beautiful you look!
9.How well you speak!
10.How you’ve grown!
11.How tall you have grown!
12.How beautifully she dances!
13.How clever she is !
14.How quickly he speaks!
15.How well everyone played!
16.How wonderful you look!
17.How nice it is!
18.How pretty she is!
19.How kind she is!
20.How handsome he is!
TT
1
Gọi biểu thức trên là A, ta có:
�=12⋅15+115⋅3+13⋅21+121⋅4+...+187⋅90A=2⋅151+15⋅31+3⋅211+21⋅41+...+87⋅901
13�=132⋅15+1315⋅3+133⋅21+1321⋅4+...+1387⋅9013A=2⋅1513+15⋅313+3⋅2113+21⋅413+...+87⋅9013
13�=12−115+115−13+13−121+121−14+...+187−19013A=21−151+151−31+31−211+211−41+...+871−901
13�=12−19013A=21−901
13�=224513A=4522
�=2245x13=22585A=45x1322=58522