Baidu has the resolution of this problem, I can't understand it Let α ∈ (0, half π), the equation x & # 178; sin α + Y & # 178; Cos α = 1 denote the ellipse with focus on the y-axis, then the value range of α is the most detailed solution to this problem

Baidu has the resolution of this problem, I can't understand it Let α ∈ (0, half π), the equation x & # 178; sin α + Y & # 178; Cos α = 1 denote the ellipse with focus on the y-axis, then the value range of α is the most detailed solution to this problem

For an ellipse with focus on Y axis, the denominator under y ^ 2 is greater than that under x ^ 2. 1 / cosa > 1 / Sina, then cos α < sin α because α is positive in the first quadrant cos α and sin α, and then there is a formula graph, where π / 4 to 5 π / 4 is sin α > cos α, and the rest is sin α < cos α. Because α is in the first quadrant, α ∈ (π / 4, π / 2)

Let R be the solution set of the inequality (A-2) about X multiplied by the square of X + 2 (A-2) x-4 〈 0, and find the value range of A

(a-2)X²＋2(a-2)x-4

Through the ellipse C: x ^ 2 / 8 + y ^ / 4 = 1 point P (x0, Y0) to the circle O: x ^ 2 + y ^ 2 = 4, two tangent lines PA, Pb, a and B are introduced as tangent points, such as the intersection of line AB with X axis and Y axis at M and n
(2) Solving the equation of line AB (expressed by x0, Y0)
The line AB is the common chord of the circle (with OP as the diameter) where the four points of the circle O and the OAPB share. The solution of the common chord of the two circles only needs to subtract the square terms of X and Y from the equations of the two circles (x ^ 2 + y ^ 2 = 4 minus x (x-x0) + y (y-y0) = 0), and the line is x0x + y0y = 4
Four points in a circle, I understand. How does the equation x (x-x0) + y (y-y0) = 0 come from?
Do you know that if four points are concentric and one point is concentric, you can use this equation to set a circle?

The equation represents an equation that passes through both the origin and the given point