Question

tìm x,biết: 

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

 

Answer 1

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

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

=> 2.( 1 + 2 + 3 + 4 + ... +x ) = 210

=> 2. [ x.( x+ 1) /2 ] = 210

 => x. ( x + 1 ) = 210

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

Vậy x = 14

Answer 2

\(2+4+6+8+...+2.x=210\)

\(=2.1+2.2+2.3+...+2.x=210\)

\(=2.\left(1+2+3+....+x\right)=210\)

=\(2.\left[\frac{x.\left(x+1\right)}{2}\right]=210\)

\(\Rightarrow x.\left(x+1\right)=14.\left(14+1\right)\)

\(=x.\left(x+1\right)=210\)

=> x=14

Answer 3

Ta có: 2+4+6+...+2x = 210

=> 2.1+2.2+2.3+....+2.x = 210

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

=> 2.[x.(x+1)/2]         = 210

=>     x.(x+1)             = 210

Mà x.(x+1) = 14.15

=> x = 14