Let θ∈ (0, π / 2), then the curve represented by the equation x ^ 2cos θ + y ^ 2Sin θ = 1 is? A ellipse B circle C ellipse or circle D straight

Let θ∈ (0, π / 2), then the curve represented by the equation x ^ 2cos θ + y ^ 2Sin θ = 1 is? A ellipse B circle C ellipse or circle D straight

x^2/(1/cosθ)+y^2/(1/sinθ)=1
θ∈（0,π/2）
Then θ = π / 4, 1 / cos θ = 1 / sin θ
This is the circle
It is an ellipse when 1 / cos θ and 1 / sin θ are greater than 0
Choose C

Given that point a (1,0), P is any point on the curve X = 2cos θ, y = 1 + Cos2 θ (θ∈ R), let the distance from P to the straight line L: y = - 1 / 2 be D, then the minimum value of PA + D is

The parametric equation is transformed into the equation y = x ^ 2 / 2 in the rectangular coordinate system, which is a parabola with the opening upward. The Quasilinear equation is l, and the focus f (0,1 / 2). Then the distance from P to L is PF, so the minimum distance of PA + D is fa = √ 5 / 2

Given the trajectory equation of the midpoint of the chord cut by the circle x ^ 2 + y ^ 2 = 25 by the straight line L passing through the point m (1,2)

Let the linear equation be: y = KX + B substituted into the circle equation, then, X & sup2; + (KX + b) & sup2; = 25, expand, (1 + K & sup2;) x & sup2; + 2bkx + B & sup2; - 25 = 0, let the two solutions of the equation be: x1, X2, then, X1 + x2 = - BK / (1 + K & sup2;) change the linear equation into: x = (y-b) / K, substituted into the circle equation

Given the curve C x ^ 2 / 4 + y ^ 2 / 9 = 1, the straight line L x = 2 + t y = 2-2t, write the parameter equation of curve C, the straight line
The common equation of the equation is to find the maximum and minimum value of PA at point a through the intersection l of any point P and L on the curve C with an angle of 30 & #

c: X = 2cos θ, y = 3sin θ; l: Y-2 = - 2 (X-2); K1 = - 2, K2 = tan30 ° = √ 3 / 3, - >, k = (K1 + K2) / (1-k1 * K2) = 8-5 √ 3, x ^ 2 / 4 + y ^ 2 / 9 = 1, - >, y '= - 9x / 4Y = k, - >, XP =, YP =, the known data can not get a simple solution, check whether the known data have problems