I assume you happen to mean, what is the formula for a circle.
The formula is:
A=
Don't understand this either
Answer:

Step-by-step explanation:
=
We need a common denominator before we can add.
+
means the same thing as
-
= 
DONT GO TO THAT LINK ITS A VIRUS AND ITS A BOT
Given:
The point is B(2,5) and its image is B'(1,2.5).
To find:
The scale factor.
Solution:
If a figure is dilated by the scale factor k and (0,0) is the center of then the scale factor is defined as

x-coordinate of point B' is 1 and the x-coordinate of point B is 2. So, the scale factor is

Therefore, the scale factor is
and the correct option is D.