[题目]已知方程(1)当t为参数,θ为常数(θ≠kπ/2,k∈Z)时,化为普通方程,并说明是何曲线;(2)当θ为参数,t为常数(t≠0)时,化为普通方程,并说明是何曲线;(3)求证:这两条曲线有共同焦点。这一道习题,作为曲线的参数方程与普通方程互化的典型问题,深受人们青睐,频频出现于各种数学报 [topic] Known equation (1) When t is a parameter and θ is a constant (θ≠kπ/2, k∈Z), it is converted into an ordinary equation, and it describes what curve; (2) When θ is a parameter, t When it is a constant (t ≠ 0), it is converted into an ordinary equation, and it describes what curve it is; (3) Verification: Both curves have common focus. This problem, as a typical problem of the inter-relation between the parametric equation of the curve and the ordinary equation, is favored by people and frequently appears in various journals.