@nihal.mehta
Stated: The NOT (X) gate is equivalent to the U3(pi, pi, 0, ) gate.
I tried that in the U3 matrix, but I got a different result. Should it be U3(pi, 0, pi) ?
BTW, I really like your book. Your style of explanation, especially with the qubelets works well for me. Then when you tie it all together with the Bloch sphere and the matrix algebra gives 3 ways to look at it. Seeing it from 3 different aspects helps to cement the understanding.