Question

Tìm X :
1 + 2 + 3 + ... + X = 210
Giúp mình nhanh nhé

 

Answer 1

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

=>x(x+1)=420

=>x=14

Answer 2

`1+2+3+.........+x=210`

`x xx (x+1):2=210`

`x xx (x+1) =420`
Vậy` x=20`

Answer 3

(X+1)xX:2=210
=> (X+1)X=420
=> X^2+X = 420
 => X=20