To find the diameter you divide the circumference by pi(3.14) and to find radius you divide the diameter by 2 since the radius is half of the diameter.
That is not always true because think of it as ab=8 that is just like saying (8)(8)=8 which is absolutely false. If anything it can be either a or be equals 8 while the other variable is 1. (1)(8)=8 or (8)(1)=8