  The distribution of a time integral of geometric Brownian motion is not well
understood. To price an Asian option and to obtain measures of its dependence
on the parameters of time, strike price, and underlying market price, it is
essential to have the distribution of time integral of geometric Brownian
motion and it is also required to have a way to manipulate its distribution. We
present integral forms for key quantities in the price of Asian option and its
derivatives ({\it{delta, gamma,theta, and vega}}). For example for any $a>0$
$\mathbb{E} [ (A_t -a)^+] = t -a + a^{2} \mathbb{E} [ (a+A_t)^{-1} \exp
(\frac{2M_t}{a+ A_t} - \frac{2}{a}) ]$, where $A_t = \int^t_0 \exp (B_s -s/2)
ds$ and $M_t =\exp (B_t -t/2).$
