If $1+2x+3x^2 + \dotsb=9$, find $x$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
If $1+2x+3x^2 + \dotsb=9$, find $x$.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{2}{3}
If $1+2x+3x^2 + \dotsb=9$, find $x$.
train