Folks,

I'm trying to evaluate (1/2)sin(2arcsec(-8)) where arcsec is the inverse secant. The answer is supposed to be 3sqrt(7)/128. I'll post a sketch of what I've done so far.

Question

Folks,

I'm trying to evaluate (1/2)sin(2arcsec(-8)) where arcsec is the inverse secant. The answer is supposed to be 3sqrt(7)/128.  I'll post a sketch of what I've done so far.

anonymous · Answer

|dw:1343155955658:dw|. I'm not sure how deal with the 2arcsec(-8)

anonymous · Answer

$$arcsec(-8) = \frac{\pi}{2}+\arcsin(1/8)$$
If that sounds weird, draw a sketch and you might notice why.

anonymous · Answer

It might be an equivalence, but how  does that become 3sqrt(7)/128?

anonymous · Answer

Is there an identity I'm missing?