imc

It is often easier to first draw a diagram for such a problem. Sometimes, it may also be easier to think of the problem in 2D. Take a cross section of the pyramid through the apex and two points from the base that are opposite to each other. Place it in two dimensions. The dimensions of this triangle are $1,1,$ and $\sqrt{2}$ because the sidelengths of the pyramid are $1$ and the base of the triangle is the diagonal of the pyramid's base. This is a $45-45-90$ triangle. Also, we can let the dimensions of the rectangle be $s$ and $s\sqrt{2}$ because the longer length was the diagonal of the cube's base and the shorter length was a side of the cube. The two triangles on the right and left of the rectangle are also $45-45-90$ triangles because the rectangle is perpendicular to the base, and they share a $45^{\circ }$ angle with the larger triangle. Therefore, the legs of the right triangles can be expressed as $s.$ Now we can just use segment addition to find the value of $s.$ $$\sqrt{2}=s+s\sqrt{2}+s=2s+s\sqrt{2}=(2+\sqrt{2})s$$ $$s=\frac{\sqrt{2}}{2+\sqrt{2}}=\frac{1}{\sqrt{2}+1}=\frac{\sqrt{2}-1}{2-1}=\sqrt{2}-1$$ The volume of the cube is $s^3=(\sqrt{2}-1{)}^3=(\sqrt{2}-1)(3-2\sqrt{2})=3\sqrt{2}-3-4+2\sqrt{2}=\boxed{5\sqrt{2}-7}$