tana = √5/x
sina = √5/√(x² + 5)
cosa = (√2)x/4
tana = sina/cosa
√5/x = [√5/√(x² + 5)]/[(√2)x/4]
x = ±√3
a为第二象限角,x < 0,x = -√3
sina = (√10)/4
tana = √5/(-√3) = -(√15)/3