regular tetrahedron
Volume V=√3/4a? *√6/3a* 1/3=√2/ 12a？
V is also equal to √3/4a? *R* 1/3*4
The solution is r = √ 6/3 * a.
In the same way; In a similar way
Right pyramid
Volume V=a? *√2/2a* 1/3=√2/6a？
V is also equal to √3/4a? *R* 1/3*3
The solution is r = 2 √ 6/9 * a.
Then forget it.