1+2+3+...+x=45
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Sửa đề:\(\dfrac{1}{21}+\dfrac{1}{28}+\dfrac{1}{36}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{2}{9}\)
\(\dfrac{2}{42}+\dfrac{2}{56}+\dfrac{2}{72}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{2}{9}\)
\(\dfrac{2}{6.7}+\dfrac{2}{7.8}+\dfrac{2}{8.9}+...+\dfrac{2}{x\left(x+1\right)}=\dfrac{2}{9}\)
\(2\left(\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+...+\dfrac{1}{x\left(x+1\right)}\right)=\dfrac{2}{9}\)
\(\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+...+\dfrac{1}{x}-\dfrac{1}{x+1}=\dfrac{2}{9}:2\)
\(\dfrac{1}{6}-\dfrac{1}{x+1}=\dfrac{2}{9}.\dfrac{1}{2}\)
\(\dfrac{1}{6}-\dfrac{1}{x+1}=\dfrac{2}{18}\)
\(\dfrac{1}{x+1}=\dfrac{1}{6}-\dfrac{1}{9}\)
\(\dfrac{1}{x+1}=\dfrac{3}{18}-\dfrac{2}{18}\)
\(\dfrac{1}{x+1}=\dfrac{1}{18}\)
\(x+1=18\)
\(x=18-1\)
\(x=17\)
`#H`
k)
<=> \(\left(\dfrac{7}{2}-2x\right):\dfrac{10}{3}=\dfrac{22}{3}\)
<=> \(\dfrac{7}{2}-2x=\dfrac{22}{3}\times\dfrac{10}{3}=\dfrac{220}{9}\)
=> \(2x=\dfrac{7}{2}-\dfrac{220}{9}=\dfrac{-377}{18}\)
=> \(x=\dfrac{-377}{18}:2=\dfrac{-377}{36}\)
j , \(720:\left[41-\left(2x+5\right)\right]=40\)
\(41-\left(2x+5\right)=720:40\)
\(41-\left(2x+5\right)=18\)
\(2x+5=41-18\)
\(2x+5=23\)
2x =28
=> x=9
k,
\(\left(3\dfrac{1}{2}-2x\right):3\dfrac{1}{3}=7\dfrac{1}{3}\)
\(\left(\dfrac{7}{2}-2x\right):\dfrac{10}{3}=\dfrac{22}{3}\)
\(\dfrac{7}{2}-2x=\dfrac{22}{3}\cdot\dfrac{10}{3}\)
\(\dfrac{7}{2}-2x=\dfrac{220}{9}\)
\(2x=\dfrac{7}{2}-\dfrac{220}{9}\)
\(2x=-\dfrac{377}{18}\)
x=\(-\dfrac{377}{36}\)
Mình nghĩ câu cuối đề sai phải là 2/x(x+1) chứ nhỉ ?
`2/3<(1/6+2/15+3/40+4/96):5 xx x<5/6`
`2/3<(1/6+2/15+3/40+1/24):5 xx x<5/6`
`2/3<(20/120+48/120+9/120+5/120):5 xx x<5/6`
`2/3<82/120:5xx x<5/6`
`2/3<41/60:5 xx x<5/6`
`2/3<41/60xx1/5 xx x <5/6`
`2/3<41/300 xx x<5/6`
`2/3<(41x)/300<5/6`
`200/300<(41x)/300<25/300`
`200<41x<250`
`41x in {205;246}`
`x in {5;6}`
Ta có `:`
`x/2 = y/3 -> x/10 = y/15(1)`
`y/5 = z/7 -> y/15 = z/21(2)`
Từ `(1)(2)` ta có dãy tỉ số bằng nhau `: x/10=y/15 = x/21`
Áp dụng tính chất dãy tỉ số bằng nhau ta được `:`
`x/10=y/15 = x/21` `= ( x + y + z )/( 10 + 15 + 21 ) = 92/46 = 2`
`->` `{(x/10=2),(y/15=2),(z/21=2):}`
`-> {(x=20),(y=20),(z=42):}`
Vậy `....`
x/2 = y/3
=> x.5/2.5 = y.5/3.5
=> x/10=y/15
y/5 = z/7
=> y.3/5.3= z.3/7.3
=> y/15 = z/21
x/10=y/15; y/15=z/21
Nên suy ra ta được :
x/10= y/15=z/21
x+y+x/10+15+21
=> 92/46= 2
=> x= 10.2= 20
=> y= 15.2=30
=> z= 21.2=42
1. A = {W,I,F}
2.
a) B = {tháng 4; tháng 6; tháng 9; tháng 11}
b) quý mấy bạn?
1 + 2 + 3 + ... + x = 45.
( x + 1 ) . x : 2 = 45.
( x + 1 ) . x = 90.
( x + 1 ) . x = 10 . 9.
Vậy x = 9.