Question

Suppose the diameter of a circle is 10. What is its area?

Answer 1

Well hello again :3.  This is also quite simple.  The diameter of a circle is 2 times the radius.  So we can refit this into the equation and say that our new formula using diameter is $pi(d^2/2)$.  Now we can say that 10/2 is 5 and $5^2$ is 25.  Therefore your answer is $25pi$ or 78.54.  

Answer 2

To find area you must use the formula A= pi*radius. So the radius is always 1/2 of the diameter. 10/2 is 5.So your radius is 5. multiply 5 by 3.14, to get the area of 78.54