a)2+4+6+..+2x=210

=>2*1+2*2+...+2*x=210

=>2(1+2+3+...+x)=210

=>2[x(x+1)/2]=210

=>x*(x+1)=210

hay x*(x+1)=14(14+1)

vậy x=14

c)x+(x+1)+(x+2)+...+(x+2010)

=>(x+x+x+...+x)+(1+2+3+...+2010)=2029099

=>2011x           + 2021055         =2029099

     2011x                                     =2029099-2021055

       2011x                                   =8044

         x                                          =8044 /2011

          x                                          =4

k cho mình nhé

2 + 4 + 6 + 8 + ... + 2x = 210

2( 1 + 2 + 3 + 4 + ... + x ) = 210

1 + 2 + 3 + 4 + ... + x = 210 : 2

1 + 2 + 3 + 4 + ... + x = 105

x . ( x + 1 ) : 2 = 105

x . ( x + 1 )  = 105 . 2

x . ( x + 1 ) = 210

Mà 14 . 15 = 210

=> x = 14

2+4+6+8+...+28=210

Ta có: 2 + 4 + 6 + 8 +......+ 2x = 210

<=> (2 + 4 + 6 + ...... + 2x) : 2 = 210 : 2

<=> 1 + 2 + 3 + ...... + x = 105

Số số hạng của dãy trên là :
                (x - 1) + 1 = x (số)

Theo baiof ra ta có: x.(x + 1) / 2 = 105

=> x.(x + 1) = 210

=> x.(x + 1) = 14.15

=> x = 14

a)

\(x+\left(x+1\right)+\left(x+2\right)+...+\left(x+2010\right)=2029099\\ 2011.x+\left(1+2+3+...+2010\right)=2029099\\ 2011.x+2021055=2029099\\ 2011.x=2029099-2021055\\ 2011.x=8044\\ x=8044:2011\\ x=4\)

b)

\(2+4+6+...+2x=210\\ 2.\left(1+2+3+...+x\right)=210\\ 1+2+3+...+x=210:2\\ 1+2+3+...+x=105\\ \dfrac{x.\left(x+1\right)}{2}=105\\ x.\left(x+1\right)=105.2\\ x\left(x+1\right)=210\\ x.\left(x+1\right)=14.15\\\Rightarrow x=14\)

a) x+(x+1)+(x+2)+...+(x+2010)=2029099

x+x+1+x+2+...+x+2010=2029099

2011x+[(2010+1).2010:2]=2029099

2011x+2021055=2029099

2011x=2029099-2021055

2011x=8044

x=8044:2011

x=4

Vậy x=4.

b) 2+4+6+8+...+2x=210

(2x+2)*14:2=210

(2x+2)*7=210

2x+2=210:7

2x+2=30

2x=30-2

2x=28

x=28:2

x=14

Vậy x=14.

a) \(\dfrac{2}{5}\cdot x=\dfrac{1}{2}+\dfrac{6}{8}\)

    \(\dfrac{2}{5}\cdot x=\dfrac{5}{4}\)

          \(x=\dfrac{5}{4}\div\dfrac{2}{5}\)

          \(x=\dfrac{25}{8}\)

b) \(\dfrac{20}{7}-x=\dfrac{19}{7}\div\dfrac{3}{2}\)

    \(\dfrac{20}{7}-x=\dfrac{19}{7}\cdot\dfrac{2}{3}\)

    \(\dfrac{20}{7}-x=\dfrac{38}{21}\)

             \(x=\dfrac{20}{7}-\dfrac{28}{21}\)

             \(x=\dfrac{22}{21}\)

a) x=25/8

b)=22/21

a) x=25/8

b)=22/21

=> 2011x + 1 + 2 +... + 2010 = 2029099

=> \(2011x+\frac{2010.2011}{2}=2029099\)

\(\Rightarrow2011x=2029099-2021055=8044\)

=> x = 4

=> 2.1+2.2+2.3+2.4+...+2.x=210

=> 2(1+2+3+...+x) = 210

=> 1 + 2 +3 +...+ x = 105

=> \(\frac{x\left(x+1\right)}{2}=105\Leftrightarrow x\left(x+1\right)=210=14.15\)

=> x = 14

\(a,\dfrac{3}{8}=\dfrac{6}{x}\\ \Rightarrow x=6:\dfrac{3}{8}\\ \Rightarrow x=16\\ b,\dfrac{1}{9}=\dfrac{x}{27}\\ \Rightarrow x=\dfrac{1}{9}.27\\ \Rightarrow x=3\\ c,\dfrac{4}{x}=\dfrac{8}{6}\\ \Rightarrow x=4:\dfrac{4}{3}\\ \Rightarrow x=3\\ d,\dfrac{3}{x-5}=\dfrac{-4}{x+2}\\ \Rightarrow3\left(x+2\right)=-4\left(x-5\right)\\ \Rightarrow3x+6=-4x+20\\ \Rightarrow3x+6+4x-20=0\\ \Rightarrow7x-14=0\\ \Rightarrow7x=14\\ \Rightarrow x=2\)

a: =>6/x=3/8

hay x=16

b: =>x/27=1/9

nên x=3

c: =>4/x=4/3

nên x=3

d: =>3/x-5=-4/x+2

=>3x+2=-4x+20

=>7x=18

hay x=18/7