A custodian carries a large key ring. He measures it and calculates that it has a circumference of 25.12 centimeters. What is the key ring's radius?

By definition of , we have

and we deduce

Now, recall that the diameter is twice the radius:

and solve for :

So, divide the circumference by and you'll find the radius.

for finding the midpoint of any given 2 pair of points simply add the respective x coordinates with other x coordinate and the y coordinate with other y coordinate and divide them by 2

in your question it will be :

(3+1)/2 for the x coordinate of mid point i.e 2

[8+(-4)]/2 for the y coordinate of midpoint i.e 2

so coordinates of mid point of points (3,8) and (1,-4) will be (2,2)