how to deduce cos rule, from sine rule??????
In short, you don't. To derive the Cosine Rule, you draw in the height of your triangle and apply Pythagoras' Theorem to the resulting right-angle triangle.
Or in the case of an obtuse-angle triangle...
We use the fact that $\displaystyle \displaystyle \begin{align*} \sin{ \left( 180^{\circ} - \theta \right) } = \sin{(\theta)} \end{align*}$ and $\displaystyle \displaystyle \begin{align*} \cos{ \left( 180^{\circ} - \theta \right) } = -\cos{(\theta)} \end{align*}$, then by Pythagoras we have
$\displaystyle \displaystyle \begin{align*} \left[ a - b\cos{(\theta)} \right] ^2 + \left[ b\sin{(\theta)} \right] ^2 = c^2 \end{align*}$
which upon simplification gives the same result.