tanα*[cos（3π-α）-sin（5π+α）]
=tanα*[cos(2π+π-α)-sin(4π+π+α）]
=tanα*[cos(π-α)-sin(π+α)]
=tanα*[-cosα+sinα]
=sinα(-cosα+sinα)/cosα
=-sinα+(sinα)^2/cosα,
α是第四象限角,cosα=4/5,
原式=3/5+(9/25)/(4/5)
=3/5+9/20
=21/20.