You serious? Radius and height of the cone is also 5 cm. So, it's (1/3)(pi)(5^2)(5), following the formula for cone volume, which is 1/3pi(radius)^2(height). Do the rest yourself.
if you have it set up with the cones radius of 5 that is not maximizing the area of the of the cone. To find it wouldn't you take the derivative of volume of a cone formula, but replace the r and the h with h=5+5cos(x) and r=5sin(x).
Wait nevermind. I thought it said equilateral or something. It's a related rates problem. You gotta relate the radius to the volume of the cone, plug it into the cone volume equation and find that maximum. I don't feel like explaining more because it's been a year and I have stats homework to finish.