a, X-3/4-3/28-3/70-3/130-3/208= 1/6
b, (1/15+1/35+1/63)xX =1
C, 242/363+1616/2121=2/7xX
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` 242/363 + 1616/2121 = 2/7 xxy`
`2/7 xxy= 2/3 + 16/21`
`2/7 xxy= 14/21 +16/21`
`2/7 xxy= 30/21`
`y=10/7 : 2/7`
`y=10/7 xx 7/2`
`y=70/14`
`y=5`
__
` (y + 1/4) + (y + 1/16) + (y + 1/16) =2`
`(y+y+y)+(1/4 + 1/16+1/16)=2`
`3y + (4/16 +1/16 +1/16)=2`
`3y + 6/16=2`
`3y=2-6/16`
`3y= 32/16-6/16`
`3y= 26/16`
`y=26/16 : 3`
`y=26/48`
`y=13/24`
\(a,\dfrac{242}{363}+\dfrac{1616}{2121}=\dfrac{2}{7}\times y\)
\(\dfrac{2}{7}\times y=\dfrac{2\times121}{3\times121}+\dfrac{16\times101}{21\times101}\)
\(\dfrac{2}{7}\times y=\dfrac{2}{3}+\dfrac{16}{21}\)
\(\dfrac{2}{7}\times y=\dfrac{14}{21}+\dfrac{16}{21}\)
\(\dfrac{2}{7}\times y=\dfrac{30}{21}\)
\(\dfrac{2}{7}\times y=\dfrac{10}{7}\)
\(y=\dfrac{10}{7}:\dfrac{2}{7}\)
\(y=\dfrac{10}{7}\times\dfrac{7}{2}\)
\(y=5\)
\(---\)
\(b,\left(y+\dfrac{1}{4}\right)+\left(y+\dfrac{1}{16}\right)+\left(y+\dfrac{1}{16}\right)=2\)
\(\left(y+y+y\right)+\left(\dfrac{1}{4}+\dfrac{1}{16}+\dfrac{1}{16}\right)=2\)
\(3\times y+\left(\dfrac{4}{16}+\dfrac{2}{16}\right)=2\)
\(3\times y+\dfrac{6}{16}=2\)
\(3\times y+\dfrac{3}{8}=2\)
\(3\times y=2-\dfrac{3}{8}\)
\(3\times y=\dfrac{16}{8}-\dfrac{3}{8}\)
\(3\times y=\dfrac{13}{8}\)
\(y=\dfrac{13}{8}:3\)
\(y=\dfrac{13}{8}\times\dfrac{1}{3}\)
\(y=\dfrac{13}{24}\)
#\(Toru\)
a) n(n+2)
b) (3n-2)3n
c) ( 1) 1 n n 2
d) 1+n2 e) n(n+5)
f) (3n-2)(3n+1)
g) n ( n 3) 2 n n
h) ( 1)( 2) 2
i) n ( n 1)( n 2)
Dãy số có 2 chữ số chia hết cho 3 là:[12,15,....,99]
Khoảng cách của từng số hạng là 3
Số số hạng là: (99-12):3+1=30(số)
Vậy có 30 số có 2 chữ số chia hết cho 3
a) Ta thấy :
U1 = 1 . 3 ; U2 = 2 . 4 ; U3 = 3 . 5 ; ... ; Un = n . ( n + 2 )
c) U1 = 1 ; U2 = 1 + 2 ; U3 = 1 + 2 + 3 ; U4 = 1 + 2 + 3 + 4 ; U5 = 1 + 2 + 3 + 4 + 5 ; ... : Un = 1 + 2 + 3 + ... + n
d) 2 + 3 = 5 ; 5 + 5 = 10 ; 10 + 7 = 17 ; 17 + 9 = 26 ; ...
f) 4 = 1 . 4 ; 28 = 4 . 7 ; 70 = 7 . 10 ; 130 = 10 . 13 ; 208 = 13 . 16 ; ...
g) 2 + 3 = 5 ; 5 + 4 = 9 ; 9 + 5 = 14 ; 14 + 6 = 20 ; ...
i) 2 + 6 = 8 ; 8 + 12 = 20 ; 20 + 20 = 40 ; 40 + 30 = 70 ; ...
1/3+1/15+1/35+1/63+1/99+1/143+1/195
=1/1*3+1/3*5+1/5*7+1/7*9+1/9*11+1/11*13+1/13*15
suy ra 2(1/1*3+1/3*5+1/5*7+1/7*9+1/9*11+1/11*13+1/13*15)
=2/1*3+2/3*5+2/5*7+2/7*9+2/9*11+2/11*13+2/13*15
=1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13+1/13-1/15
=1-1/15
=14/15
a=14/15 chia 2=7/15