Step One
Find the area of the whole pizza.
Area = pi*r^2
r = d/2
d = 18
r = 18/2 = 9
Area = pi * r^2
Area = pi * 9^2
Area = 81 pi
Part Two
If you cut this into 10 slices each slice will have 1/10 of the total area.
Area whole pizza = 81*pi
Area of 1 slice = (1/10) * 81 pi
Area of 1 slice = 8.1*pi