No calculator: Final Destination

sin(x)*cos(x)=c, where c is a constant between 0 and 1.

Without a calculator, how do I do this?  Until now, I just graphed that shit.  But now I'm not allowed to, so I come to you for help.

I am not sure about this but usually the 'other' solution for arcsine is "pi - arcsin(...etc)"

So in this case, x = arcsin(2c)/2 and x = pi/2 - arcsin(2c)/2

Gimmie a sec to verify.