Our good friend geometry is, apparently, not Blueicus' friend:
For a circle, A = pi x r^2, not pi x d^2. Let's see what that means in this case:
(I'm going to do some rounding off to keep things simple)
For the 8" cake pan, A = pi x 4 x 4 = (approx) 50 sq in
For the 9" cake pan, A = pi x 4.5 x. 4.5 = (approx) 64 sq in.
Arithmetic and algebra are also sulking.
3 pans x 50 sq in = 150 sq in
2 pans x 64 sq in = 128 sq in -- but think of it as 130 sq in just to keep things simple. (I warned you about the rounding off, dont get smug) So, the ratio is 15:13
Assuming the pans are 2" deep, the total volume for the 8" pans is 300 cu in, and for the 9" pans it's 260 cu in. However the actual depth is not important, as long as the depth of the 9" pans is equal or greater than the depth of the 8" pans. Returning to our equal depth assumption, our 15:13 ratio isn't really all that different than blueicus' 6:5 ratio. Of course it isn't. The ratio of a given squared diameter to the squared radius calculated from the diameter, is always 4:1, and since it's constant it cancels out. Better to be lucky than good, eh? :crazy:
You're either going to have to cut down the recipe by a bit, or hope the recipe fits in your 9" pans. As izbnso pointed out, most recipes allow a little room, the whole recipe will probably fit. However, combining her prediction with your actual number of pans, 3 x 3-1/2 cups = 10-1/2 cups which is darn close to the 10-11 cups she predicts will fill 2 x 9" pans. This should make you feel better. I know it does me. ;)
Presumably Norma got mixed up and thought the problem was not enough batter. Considering that you're worried about whether or not you have sufficient volume for the recipe, the suggestion to put more batter in one pan doesn't make sense. Splitting 9" dia cakes is not easy. It's not the cutting, it's the moving and stacking. Unless you have spatulas the right size and have done this sort of thing before, I'd suggest skipping it for now and learning with smaller cakes. Also, you'd be icing 4 x 64 sq in surfaces instead of 3 x 50 sq in surfaces, plus a bigger area on the side, and would need (approx) twice as much frosting.
Hope this clarifies,