tìm x: 2+4+6+.....+2x=210

Question

2+4+6+...+2x = 210 

Answer 1

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

<=>  {[(2x-2):2+1]:2}.(2x+2) =210 

<=> {[2(x-1):2+1]:2}.2(x+1) =210 

<=> [(x-1)+1]:2.2(x+1) =210

<=> (x-1+1)(x+1) =210

<=> x(x+1) =210

Vì 14.15=210 nên x=14

Answer 2

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

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

<=>2.    = 210

<=> 

<=> x = 14

hoặc x = -15

Vậy...