Cho
A=1/3.8+1/8.13+1/13.18+..........+1/33.38
B=1/3.10+1/10.17+1/17.24+.......+1/31.38
tính tỷ số A/B
cho lời giải
Tính A=1/3.8 + 1/8.13 + 1/13/8 + ... + 1/33.38 B=1/3.10 + 1/10.17 + 1/17.24 + 1/24.31 + 1/31.38
5A = 5/3.8 + 5/8.13 + 5/13.18 + ... + 5/33.38
= 1/3 - 1/8 + 1/8 - 1/13 + ... + 1/33 - 1/38
= 1/3 - 1/38
5A = 35/114
=> A = 35/114 : 5 = 7/144
7B = 7/3.10 + 7/10.17 + ... + 7/31.38
= 1/3 - 1/10 + 1/10- 1/17 + .. + 1/31 - 1/38
= 1/3 - 1/38
B = 35/144 : 7 = 5/144
VẬy A> B
Tính A=1/3.8 + 1/8.13 + 1/13/8 + ... + 1/33.38 B=1/3.10 + 1/10.17 + 1/17.24 + 1/24.31 + 1/31.38
A=1/3.8+1/8.13+1/13.18+...+1/33.38
5A=5/3.8+5/8.13+5/13.18+...+5/33.38
5A=1/3-1/8+1/8-1/13+1/13-1/18+...+1/33-1/38
5A=1/3-1/38
5A=38/114-3/114
5A=35/114
A=35/114:5
A=7/114
B=1/3.10+1/10.17+1/17.24+1/24.31+1/31.38
7B=7/3.10+7/10.17+7/17.24+7/24.31+7/31.38
7B=1/3-1/10+1/0-1/17+1/17-1/24+1/24-1/31+1/31-1/38
7B=1/3-1/38
7B=38/114-3/114
7B=35/114
B=35/114:7
B=5/114
Tìm x:( 1 phần 3.8+ 1 phần 8.13+ 1 phần 13.18+…+1 phần 33.38)-x= 31 phần 114
mọi người ơi giúp mik với....thanks ....
A = 1/1.3 + 1/3.5 + 1/5.7 + . . . + 1/55.57
b = 1/4 +1/28 + 1/70 + ...+1/58.61
c = 2/ 3.8 + 2/8.13 + 2/13.18 + . . . + 2/498.503
~chọn 1 trong 3 câu để làm ạ~
3 câu như nhau cả thôi :v
\(A=\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+...+\frac{1}{55\cdot57}\)
\(A=\frac{1}{2}\left(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+...+\frac{2}{55\cdot57}\right)\)
\(A=\frac{1}{2}\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{55}-\frac{1}{57}\right)\)
\(A=\frac{1}{2}\left(1-\frac{1}{57}\right)\)
\(A=\frac{1}{2}\cdot\frac{56}{57}\)
\(A=\frac{28}{57}\)
Tính giá trị của biểu thức:
P=\(\frac{1}{3.10}+\frac{1}{10.17}+\frac{1}{17.24}+...+\frac{1}{73.80}-\frac{1}{2.9}-\frac{1}{9.16}-\frac{1}{16.23}-\frac{1}{23.30}\)
Ta có:
P=\(\frac{1}{3.10}\)+\(\frac{1}{10.17}\)+\(\frac{1}{17.24}\)+......+\(\frac{1}{73.80}\)-\(\frac{1}{2.9}\)-\(\frac{1}{9.16}\)-\(\frac{1}{16.23}\)-\(\frac{1}{23.30}\))
P=\(\frac{1}{7}\)\(\times\)(\(\frac{7}{3.10}\)+\(\frac{7}{10.17}\)+\(\frac{7}{17.24}\)+......\(\frac{7}{73.80}\)-\(\frac{7}{2.9}\)-\(\frac{7}{9.16}\)-\(\frac{7}{16.23}\)-\(\frac{7}{23.30}\))
P=\(\frac{1}{7}\)\(\times\)(\(\frac{1}{3}\)-\(\frac{1}{10}\)+\(\frac{1}{10}\)-\(\frac{1}{17}\)+.....+\(\frac{1}{73}\)-\(\frac{1}{80}\)-\(\frac{1}{2}\)-\(\frac{1}{9}\)-......-\(\frac{1}{23}\)-\(\frac{1}{30}\))
P=\(\frac{1}{7}\)\(\times\)(\(\frac{1}{3}\)-\(\frac{1}{80}\))-\(\frac{1}{7}\)(\(\frac{1}{2}\)-\(\frac{1}{30}\))
P=\(\frac{1}{7}\)\(\times\)(\(\frac{1}{3}\)-\(\frac{1}{80}\)-\(\frac{1}{2}\)+\(\frac{1}{30}\))
P=\(\frac{-7}{336}\)
Bài này mk ko tính máy tính nên ko chắc đâu
Tính: \(\dfrac{1}{3.10}+\dfrac{1}{10.17}+\dfrac{1}{17.24}+...+\dfrac{1}{73.80}-\dfrac{1}{2.9}-\dfrac{1}{9.16}-\dfrac{1}{16.23}-\dfrac{1}{23.30}+\dfrac{1}{1.3}-\dfrac{1}{2.4}+\dfrac{1}{3.5}-\dfrac{1}{3.6}+...+\dfrac{1}{97.99}-\dfrac{1}{98.100}\)
Đặt biểu thức cần tính là A, ta có:
A=\(\dfrac{1}{7}\left(\dfrac{7}{3.10}+\dfrac{7}{10.17}+...+\dfrac{7}{73.80}\right)\)
Làm tg tự với những cái khác là ok
1/3.10+1/10.17+....+1/73.80-1/2.9-1/9.16-1/10.17-1/23.30 helppp me!!!
\(\dfrac{1}{7}\left(\dfrac{7}{3.10}+\dfrac{7}{10.17}+...+\dfrac{7}{73.80}-\left(\dfrac{7}{2.9}+\dfrac{7}{9.16}+...+\dfrac{7}{23.30}\right)\right)\)
\(=\dfrac{1}{7}\left(\dfrac{1}{3}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{17}+...+\dfrac{1}{73}-\dfrac{1}{80}-\left(\dfrac{1}{2}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{16}+...+\dfrac{1}{23}-\dfrac{1}{30}\right)\right)\)
\(=\dfrac{1}{7}\left(\dfrac{1}{3}-\dfrac{1}{80}-\left(\dfrac{1}{2}-\dfrac{1}{30}\right)\right)\)
\(=\dfrac{1}{7}\left(\dfrac{77}{240}-\dfrac{7}{15}\right)=\dfrac{1}{7}.\left(-\dfrac{7}{48}\right)=-\dfrac{1}{48}\)
dỗiii c-các cậu vì ko lm hộ t-tớ -((
Đặt cái đó=A
Nhân 7A vào rồi áp dụng đến hết 7/n(n+7)=1/n-1/n+7, trùng cái nào thì khử
Cho biểu thứcA= \(\frac{\frac{1}{3.8}+\frac{1}{8.13}+...+\frac{1}{33.38}}{\frac{21}{3.10}+ \frac{15}{10.15}+\frac{27}{15.24}+\frac{9}{24.27}+\frac{33}{27.38}}\)
a,Tính A
b,So snah A và B với B=\(\frac{1}{4}+\frac{1}{9}+\frac{1}{16}+...+\frac{1}{81}+\frac{1}{100}\)
Tính:A=1/7.12+1/12.17+1/17.22+...+1/52.57
B=10/8.13+10/13.18+10/18.23+...+10/253.258
\(A=\frac{1}{7\cdot12}+\frac{1}{12\cdot17}+\frac{1}{17\cdot22}+...+\frac{1}{52\cdot57}\)
\(A=\frac{1}{5}\left(\frac{5}{7\cdot12}+\frac{5}{12\cdot17}+\frac{5}{17\cdot22}+...+\frac{5}{52\cdot57}\right)\)
\(A=\frac{1}{5}\left(\frac{1}{7}-\frac{1}{12}+\frac{1}{12}-\frac{1}{17}+...+\frac{1}{52}-\frac{1}{57}\right)\)
\(A=\frac{1}{5}\left(\frac{1}{7}-\frac{1}{57}\right)=\frac{1}{5}\cdot\frac{50}{399}=\frac{10}{399}\)
\(B=\frac{10}{8\cdot13}+\frac{10}{13\cdot18}+\frac{10}{18\cdot23}+...+\frac{10}{253\cdot258}\)
\(B=\frac{10}{5}\left(\frac{5}{8\cdot13}+\frac{5}{13\cdot18}+\frac{5}{18\cdot23}+...+\frac{5}{253\cdot258}\right)\)
\(B=2\left(\frac{1}{8}-\frac{1}{13}+\frac{1}{13}-\frac{1}{18}+...+\frac{1}{253}-\frac{1}{258}\right)\)
\(B=2\left(\frac{1}{8}-\frac{1}{258}\right)=2\cdot\frac{125}{1032}=\frac{125}{516}\)
*Cái đây giải thích hơi bị " khó hiểu " :
Chỗ mẫu (12 - 7) = (17 - 12) = ... = (57 - 52) = 5
Tử là 1 , mẫu là 5 nên tử/mẫu = 1/5
Hay \(\frac{1}{5}\left(\frac{5}{7\cdot12}+\frac{5}{12\cdot17}+...+\frac{5}{52\cdot57}\right)\)
Còn bạn Trương Bùi Linh thì :
Mẫu = (13 - 8) = (18 - 13) = (23 - 18) = ... = 5
Tử là 10,mẫu là 5 => tử / mẫu = 10/5 = 2