1. Ellipse area theorem: the area of an ellipse is equal to π times the product of the major semi-axis length (a) and the minor semi-axis length (b) of the ellipse.

sin(a+b)= Sina cosb+cosa sin(a-b)= Sina cosb-sinbcosa .

3、sin(a/2)=√(( 1-cosa)/2)sin(a/2)=-√(( 1-cosa)/2)。

4. The characteristics of elements in a set: certainty, mutual difference and disorder. ?

5、? An empty set is a set without any elements. An empty set is a subset of any set and a proper subset of any non-empty set.

6、cosa * cosb =[cos(a+b)+cos(a-b)]/2 .

7、sinA+sinB = 2 sin((A+B)/2)cos((A-B)/2 cosa+cosB = 2 cos((A+B)/2)sin((A-B)/2)。