When x, y is a variable, and A, B, C, D, E, F, P, Q, R is a real number that is not equal to B, C and P, Q Center at the same time, it is not equal to zero at the same time.
When x y is variable, and A B C D,,,,,,, E F P Q R, real numbers without B C, not equal to zero at the same time, and P Q ไม่เท่ากับ center, at the same time.