>>13608473The second one is obvious if b is less than a, just pull b^3 out from a corner of a^3, then rearrange the remaining shell into a rectangular prism
The first one doesn’t matter unless grouping a and b together improves efficiency
Imagine b^3 at the corner of a^3, now place three rectangular prisms with rotational symmetry so that (a + b)^3 is formed, then remove them
