设α使第三象限角,tan(π-α)=-5/12,则cos(3π/2-α)=
tan(π-α)=-tanα=-5/12,∴tanα=5/12；于是α=π+arctan(5/12)
故cos(3π/2-α)=-sinα=-sin[π+arctan(5/12)]=sin[arctan(5/12)=5/√119=0.4583