Answer:
I would choose two 12 inch pizza if I want more pizza as two 12 inch pizzas have larger circumference than one 18 inch pizza
c=πd
circumference= pie × diameter
c=2πr
circumference= 2×pie×radius
A=πr²
Area= pie×square radius
Pie= 22/7, 3.14( depends on the question, sometimes the value of pie will be given in the question)
radius= half of diameter
To find perimeter of circle use 2πr
To find area of circle use πr² or πd
