This is based on the characteristics of the equation. When the sum of squares of two numbers is a constant value, but there is no other quantitative relationship between the two numbers, we should think of the identity sin? x+cos？ X= 1。 For example, the equation of a circle (x-a)? +(y-b)？ =r？ , can be expressed as r? Because? t+r？ Sin? t=r？ ,

Where x=a+rcost, y=b+rsint, and so is the elliptic equation x? /a？ +y？ /b？ = 1, which can be set to x = acost and y = bsint.

Where the angle t is the angle obtained by counterclockwise rotation with the X axis as the starting edge, and the range is [0,2π]. For a simple example, circle x? +y？ = 1 passes through A( 1/2, √3/2) and B( 1/2, -√3/2). It intersects the positive semi-axis of the X axis at D and the origin O, so I won't draw. Then at point A here, the angle of T is ∠ aod = π/3; At point B, the angle T is ∠BOD (obtuse angle) =5π/3 (due to counterclockwise rotation).

This method is commonly used in analytic geometry, which is equivalent to reducing the number of unknowns (because R or A and B usually cancel out in calculation), and the calculation is simpler.