Hi there!
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I believe your answer is:
18in³
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Here’s why:
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Assuming that the figure is a rectangular prism:
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Hope this helps you. I apologize if it’s incorrect.
From the left

the function is clearly approaching 5, while from the right

it is approaching -2.
I believe that it is 45 degrees. i could be wrong, so make sure to check!
Perfect squares are:
1,4,9,16,25,36,49,64,81,100,....
the sum of the digits of our biggest number is 16 so any perfect square bigger than 16 doesn't work for us
1-
1+0=1 so any number containing the digits will work(keep in mind we only will look into whole numbers because digits can't be negative or have fractions or be irrational)
thereful 10 works for our category
2-
0+4=4
1+3=4
2+2=4
22 13 31 and 40 will work two
3-
0+9
1+8
2+7
3+6
4+5
90 18 81 27 72 36 63 45 54
4-
0+16
1+15
2+14
3+13
4+12
5+11
6+10
7+9
8+8
79 97 88
so our set of numbers contain:
10 22 13 31 40 90 18 81 27 72 36 63 45 54 79 97 88