# Finding the exact value

• June 10th 2008, 07:51 PM
gabrie30
Finding the exact value
of this expression is driving me crazy...

sin(2sin^(-1)(-2/5))

anyone willing to enlighten me?
• June 10th 2008, 08:31 PM
mr fantastic
Quote:

Originally Posted by gabrie30
of this expression is driving me crazy...

sin(2sin^(-1)(-2/5))

anyone willing to enlighten me?

Let $\alpha = \sin^{-1} \left( - \frac{2}{5} \right) \Rightarrow \sin \alpha = - \frac{2}{5}$.

You want the value of $\sin (2 \alpha) = 2 \sin \alpha \cos \alpha$.

I will assume you can find $\cos \alpha$ given $\sin \alpha = - \frac{2}{5}$ ......