1 + 2 + 3 + ... + x = 105
=> (1 + x).x:2 = 105
=> (1 + x).x = 105.2
=> (1 + x).x = 210
=> (1 + x).x = 15.14
=> x = 14
Vậy x = 14
\(1+2+3+..+x=105\)
\(\Rightarrow\)\(\frac{\left(x+1\right)x}{2}=105\)
\(\Rightarrow x\left(x+1\right)=210=14\cdot15\)
Vậy x=14
1 + 2 + 3 + ... + x = 105
(x + 1) . [(x - 1) : 1 + 1] : 2= 105
(x + 1) . [(x - 1) + 1] = 105 . 2
(x + 1) . x = 210
Mà 14 . 15 = 210 => x = 14
Vậy số cần tìm là 14
\(1+2+3+...+x=105\)
\(\Rightarrow\frac{x\left(x+1\right)}{2}=105\)
\(\Rightarrow x.\left(x+1\right)=310\)
\(\Rightarrow x.\left(x+1\right)=14.15\)
\(\Rightarrow x=14\)
Vậy x = 14
