x2/16+y2/25=1,则可设x/4=sinθ、y/5=cosθ,得x=4sinθ、y=5cosθ y-3x=5cosθ-12sinθ =-(12sinθ-5cosθ) =-√(122+52)?[12sinθ/√(122+52)-5cosθ/√(122+52)] =-13?(12sinθ/13-5cosθ/13) =-13sin(θ+φ),其中...