This question can be answered by the definition of ellipse and basic inequality, and the fifth question in multiple-choice questions belongs to the basic question.

An ellipse is the trajectory of a moving point P. The sum of the distances from the moving point P to the fixed points F 1 and F2 in a plane is equal to a constant (greater than |F 1F2|), and F1and F2 are called the two focuses of the ellipse. The mathematical expression is | pf1|+pf2 | = 2a (2a > | f1F2 |).

Basic inequality is an inequality that is mainly used to find the maximum value of some functions and prove it. It means that the arithmetic mean of two positive real numbers is greater than or equal to their geometric mean. It is known that x > 0；; Y > 0, then: if the product xy is a constant value p, then x+y has a minimum value if and only if x = Y. (abbreviation: the sum of products is minimum, that is, x+y ≥ 2 xy)