formula

${\rm d}y &=& \frac{1}{x}{\rm d}x - \frac{1}{2x^2} ({\rm d}x)^2\\ &=& \frac{1}{x}(\mu x {\rm d}t + \sigma x {\rm d}W_t) - \frac{1}{2x^2}(\sigma^2 x^2 {\rm d}t)\\ &=& (\mu - \frac{1}{2}\sigma^2){\rm d}t + \sigma {\rm d}W_t$