Alright, have another one which the solutions manual is vague on.
I have to reduce \$f =(x1+x3+x4)(x1+x2'+x3)(x1+x2'+x3'+x4)\$. Right in the first step, though, the solution does something I don't understand.
They go to \$(x1+x3+x4)(x1+x2'+x3)(x1+x2'+x3+x4)(x1+x2'+x3'+x4)\$. Where did that 4th term come from? I thought we could add a sum if it was the same as one of the current terms? How are they allowed to change the x3?