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c ) 71 + 260 = \(\frac{x}{x}\)+\(\frac{140}{x}\)+260
331 = 1 + \(\frac{140}{x}\)+260
331 = \(\frac{140}{x}\)+261
331 = 140 : x + 261
140 : x = 331 - 261
140 : x = 70
x = 140 : 70
x = 2
a) ( x + 1 ) + ( x + 4 ) + ( x + 7 ) + ( x + 10 ) + ... + ( x + 28 ) = 155
\(\Rightarrow\)( x + x + x + ... + x ) + ( 1 + 4 + 7 + 10 + ... + 28 ) = 155
\(\Rightarrow\)x10 + ( SSH: ( 28 - 1 ) : 3 + 1 = 10 ) = 155
\(\Rightarrow\)x10 + ( Tổng: ( 1 + 28 ) . 10 : 2 = 145 ) = 155
\(\Rightarrow\)x10 + 145 = 155
\(\Rightarrow\)x10 = 155 - 145 = 10
\(\Rightarrow\)x = 10 : 10 = 1
vậy x = 1
b) ( x . 0,25 + 1999 ) . 2000 = ( 53 + 1999 ) .2000
\(\Rightarrow\)500x + 3998000 = 106000 + 3998000
\(\Rightarrow\)500x + 3998000 = 4104000
\(\Rightarrow\)500x = 4104000 - 3998000 = 106000
\(\Rightarrow\)x = 106000 : 500 = 212
\(71+65\cdot4=\frac{x+140}{x}+260\)
\(\Rightarrow71+260=\frac{x}{x}+\frac{140}{x}+260\)
\(\Rightarrow71=1+\frac{140}{x}\)
\(\Rightarrow\frac{140}{x}=70\)
\(\Rightarrow x=2\)
71+65*4=x+140 +260
x
=>71+260= x + 140 +260
x x
=>71=1+ 140
x
=> 71-1 = 140
x
=> 70=140:x
=>x=140:70=2
\(A=\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right)....\left(1-\frac{1}{2015}\right)\)
\(=\left(\frac{2-1}{2}\right)\left(\frac{3-1}{3}\right)\left(\frac{4-1}{4}\right)....\left(\frac{2015-1}{2015}\right)\)
\(=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.....\frac{2013}{2014}.\frac{2014}{2015}\)
\(=\frac{1}{2015}\)
Có: \(A=\frac{1}{2013}x\frac{2015}{2014}-\frac{2014}{2013}\)
\(=\frac{1}{2013}.\frac{2015}{2014}-\frac{1}{2013}.2014=\frac{1}{2013}.\left(\frac{2015}{2014}-2014\right)\)
\(\left(1-\frac{1}{2}\right)\times\left(1-\frac{1}{3}\right)\times\left(1-\frac{1}{4}\right)\times...\times\left(1-\frac{1}{2015}\right)\times\left(1-\frac{1}{2016}\right)\)
\(=\frac{1}{2}\times\frac{2}{3}\times\frac{3}{4}\times...\times\frac{2014}{2015}\times\frac{2015}{2016}\)
\(=\frac{1}{2016}\)
Giải : Ta có (1-1/2)*(1-1/3)*(1-1/4)*....*(1-1/2015)*(1-1/2016)
= 1* -(1/2+1/3+1/4+....+1/2015+1/2016)
= 1* - (1/2+1/2016 +1/3+1/2015 +...+1/1007)
= 1* -(1/2033134)
= -1/2033134
a/ x = (2015 . 2016) : 2015 =2016
b/ 503.5x + 2014 . 2013 = 106742 + 2014 . 2013 => 503.5x = 106742 => x = 212
c/ 71x + 260x = x+ 140 + 260x => 70x = 140 => x= 2
d/ x = 583 - 237 = 346
e/ x = 575 + 836 = 1411
g/ x = 3692 : 142 = 26
h/ x = 8928 : 124 = 72
i/ x = 168 . 34 = 5712
g/ x = 3692 : 142 = 26