sina=-五分之四吧?
sina=2sin(a/2)cos(a/2)
sin(a/2)=sina / 2cos(a/2)
=(-4/5) / (4√5/5)
=-√5/5
cot(a/2)=cos(a/2) / sin(a/2)
=(2√5/5) / (-√5/5)
=-2