What is the better deal? Two 9inch pizzas or one 13 inch pizza? Mathematical proof please.?

The 9 inch pizzas are just short of 64 square inches ea
and the 13 inch pizza is 132 square inches so by a total of 4 Square inches of pizza
the 13 inch is a winner ... oh well ...

assuming that for this comparison the
two 9 inch pizzas are equal in price to the one 13 inch....What is the better deal? Two 9inch pizzas or one 13 inch pizza? Mathematical proof please.?
if you like the edge then as circumference = 2piR where R = radius ( 4.5 and 6.5 )
2 x 2pi x 4.5 = 18pi %26gt; 2pi x 6.5 = 13pi so 2 x 9'' is better deal

if you like the topping then as area = pi R^2
2 x 4.5 x 4.5 x pi = 40.5pi %26lt; 6.5 x 6.5 x pi = 42.25pi so 1 x 13'' is the better deal

(pi is constant, and we're talking about pizzas anyway, so can be ignored.

PROVIDED the cost of 2 x 9'' pizzas = cost of 1 x 13'' pizza
A nine inch is 81 sq inches s two would be 162 sq inches. A 13 inch Pizza is 169 sq inches, just slightly larger than two 9 inchers. It depends on whether the one 13 or the two 9s are cheaper.What is the better deal? Two 9inch pizzas or one 13 inch pizza? Mathematical proof please.?
it depends on the price of the pizzas