D is a notation for differential. There is a d2 in your formula because in the numerator there are two differentials. The formula is thus the second derivative of a function with respect to two variables.
11 * 11 * 6 = 726 square inches
An irrational number is one with a fraction.
You have the right one selected.
The answer is 2:1
To find this take each side of ABC and its corresponding side on DEF, then simplify. You can treat ratios just like fractions, so simplify them in the same way.
20:10 becomes 2:1
12:6 becomes 2:1
16:8 becomes 2:1