Prove when

I had the same steps up towards , but then the negative sign just disappeared from the next step. Isn't the formula for ?

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- August 6th 2010, 02:32 PMchengbinWhat's wrong with my proof?
Prove when

I had the same steps up towards , but then the negative sign just disappeared from the next step. Isn't the formula for ? - August 6th 2010, 06:25 PMeumyang
From here:

to here:

is the problem, I think. That final angle of B/2 should be negative:

And of course, sine being an odd function, you bring the negative out: