2+4+6+8+.........+2x=210
2+4+6+8+....+2x =210
Ta có: 2+4+6+8+...+2x=210
\(\Leftrightarrow x^2+x-210=0\)
\(\Leftrightarrow x^2+15x-14x-210=0\)
hay x=14
2+4+6+8...+2x = 210
\(\Rightarrow\dfrac{\left(2x+2\right)\left[\left(2x-2\right):2+1\right]}{2}=210\\ \Rightarrow2\left(x+1\right)\left(x-1+1\right)=420\\ \Rightarrow x\left(x+1\right)=210=14\cdot15\\ \Rightarrow x=14\)
2+4+6+8...+2x = 210
<=> 2.1+2.2+2.3+2.4+....+2.x=210<=> 2.(1+2+3+4+...+x)=210<=>1+2+3+4+....+x= 210:2<=>1+2+3+4+....+x=105Lại có: x(x+1)/2 =105 => x(x+1) =210Mà 210 =14.15 => x= 14 hoặc x+1=15 => x=141Vậy x=14 Chúc các bạn học tốt nha!2+4+6+8+.....+2x=210
Ta có: 2+4+6+8+.....+2x=210
⇒ \(2\cdot\left(1+2+3+...+x\right)=2\cdot105\)
\(\Rightarrow1+2+3+...+x=105\)
\(\Rightarrow\dfrac{x\left(x+1\right)}{2}=105\)
\(\Leftrightarrow x^2+x-210=0\)
\(\Leftrightarrow\left(x+15\right)\left(x-14\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=-15\left(loại\right)\\x=14\end{matrix}\right.\)
Vậy x = 14
\(...\Rightarrow\left[\left(2x-2\right):2+1\right]\left(2x+2\right):2=210\)
\(\Rightarrow\left[2\left(x-1\right):2+1\right]2.\left(x+1\right):2=210\)
\(\Rightarrow\left[\left(x-1\right)+1\right]\left(x+1\right)=210\)
\(\Rightarrow x\left(x+1\right)=210\Rightarrow\left[{}\begin{matrix}x=0\\x=-1\end{matrix}\right.\)
2+4+6+8+...+2x=210
\(2+4+6+8+...+2x=210\)
\(\Rightarrow1+2+3+4+...+x=105\)
\(\Rightarrow\dfrac{x.\left(1+x\right)}{2}=105\)
\(\Rightarrow x.\left(1+x\right)=210\)
\(\Rightarrow x=14\)
Vậy x = 14.
2+4+6+8+.........+2x=210
\(2+4+6+8+...+2x=210\)
\(2\left(1+2+3+...+x\right)=210\)
\(2\left(\frac{x\left(x+1\right)}{2}\right)=210\)
\(\frac{2x\left(x+1\right)}{2}=210< =>x\left(x+1\right)=210\)
\(< =>x\left(x+1\right)=14,15\)
\(< =>x=14\)
\(2+4+6+8+...+2x=210\)
\(2\left(1+2+3+.....+x\right)=210\)
\(2\left(\frac{x\left(x+1\right)}{2}\right)=210\)
\(\frac{2x\left(x+1\right)}{2}=210\)\(\Leftrightarrow x\left(x+1\right)=210\)
\(\Leftrightarrow x\left(x+1\right)=14.15\)
\(\Rightarrow x=14\)
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2+4+6+8+...+2x=210
http://olm.vn/hoi-dap/question/123480.html
Trong dã số tự nhiên bắt đầu từ 1 thì số hạng thứ n là số n
Chi 2 vế cho 2:
1+2+3+....x=105
(1+x).x/2=105
(1+x).x=105.2=210
Hai số tự nhiên liên tiếp có tích là 210 là 14,15
Vậy x=14
2+4+6+8+............+2x=210
2 + 4 + 6 + 8 + ... + 2x = 210
=> 2.(1 + 2 + 3 + 4 + ... + x) = 210
=> 2.(1 + x).x:2 = 210
=> (1 + x).x = 210 = 15.14
=> x = 14
2+4+6+8+...+2x=210
2 + 4 + 6 + 8 + .... + 2x = 210
Số các số hạng: (2x - 2) : 2 + 1 = x số
2 + 4 + 6 + 8 +....+2x = x.(2x + 2) : 2 = (2x2 + 2x) : 2 = 2(x2 + x) : 2 = x2 + x = x(x + 1) = 210
x(x + 1) = 14*15
Vậy x = 14
2+4+6+8+...........+2x=210
\(C\text{ó}:2+4+6+8+...+2x=210\)
\(\Leftrightarrow2.1+2.2+2.3+2.4+...+2.x=210\)
\(\Leftrightarrow2\left(1+2+3+4+...+x\right)=210\)
\(\Leftrightarrow1+2+3+4+...+x=105\)
\(\Rightarrow\frac{x\left(x+1\right)}{2}=105\)
\(\Rightarrow x\left(x+1\right)=210\)
\(14.15=210\)
\(\Rightarrow x=14\)