x=5(3-z)cosθ/3,y=(3-z)sinθ,cosθ=3x/[5(3-z)],sinθ=y/(3-z),(3x)²/[5(3-z)]²+y²/(3-z)²=1,9x²/25+y²=(3-z)²,x²/25+y²/9=(3-z)²/9.
