of this expression is driving me crazy...
sin(2sin^(-1)(-2/5))
anyone willing to enlighten me?
Let $\displaystyle \alpha = \sin^{-1} \left( - \frac{2}{5} \right) \Rightarrow \sin \alpha = - \frac{2}{5}$.
You want the value of $\displaystyle \sin (2 \alpha) = 2 \sin \alpha \cos \alpha$.
I will assume you can find $\displaystyle \cos \alpha$ given $\displaystyle \sin \alpha = - \frac{2}{5}$ ......