Originally Posted by Project Manager

The question is to find the ratio of larger sized pizza to a smaller sized pizza.

Yes, the formula for area is A = pi * (d/2)^2 = pi * d^2 / 4

What is important that you need a ratio, and for this you don't need pi. The only thing you need is this: 22^2 / 18^2, because everything goes away. In fact, you can reduce it further to 11^2 / 9^2 which is 121 / 81. This is same as (11/9)^2.

It all comes to 1.4938271604938271604938271604938

Same thing slower:
Say Ab is area of big pizza, and As is area of small pizza.
db is diameter of big pizza = 22; ds is for small = 18.

Ab = pi * db^2 / 4
As = pi * ds^2 / 4

Ab/As = [pi * db^2 / 4] / [pi * ds^2 / 4] = db^2 / ds^2