Expert: Lynn Houston - 5/19/2010

Question
if a cone has a circumference of 20 and a slant height of 5, how do i find the lateral surface area.. dyingggg!!!

Answer
You need to use a couple different formulas.  The eq for lateral surface area of a cone is

sa = pi rs

In order to use that though, you need the radius of the base and all you have is the circumference, so you also need diameter = circumference/pi and know that the radius is half the diameter.

So  20/pi = 6.36
6.36/2 =  3.18 = radius.

Now, just plug everything into the first equation

sa = pi (3.18)(5)
sa = (3.14)(3.18)(5) = 49.92

You don't say what measurement the numbers represent (ie, inches, cm), but it would be the same for the answer.

Hope this helps.