1. Eccentricity of ellipse is defined as the ratio of focal length to major axis on ellipse (range: 0; 2c. The greater the eccentricity, the flatter the ellipse; The smaller the eccentricity, the closer the ellipse is to the circle.

2. The focal length of the ellipse: the distance between the focal point of the ellipse and its corresponding directrix (such as focal point (c, 0) and directrix X = A 2/c-C = B 2/c) is A 2/c.

3. The focus is on the X axis: | pf1| = a+ex | pf2 | = a-ex (f1,f2 is the left and right focus respectively).

4. The radius of ellipse passing through the right focus is r=a-ex.

5. The radius of the left focus is r=a+ex.

The properties of the focus triangle of an ellipse are:

( 1)|PF 1|+|PF2|=2a .

(2)4c？ =|PF 1|？ +|PF2|？ -2|PF 1| |PF2| cosθ.

(3) Perimeter =2a+2c.

(4) area =S=b? tan(θ/2)(∠F 1PF2=θ).