I'm reading Peter G. Hinman-Fundamentals of Mathematical Logic, I'm new with stuff like proofs, and as newbie I'm not used to proving anything, so I'm jammed in the exercises of the book of the section 1.1, I really want to solve the exercise of proving that an initial proper segment of a sentence can't be a sentence, I think this can involve induction, but I really can't tackle the problem.
Any hint is appreciated. Thanks in advance!
I posted a link to the book, but it was erased, please see the caveman comment below for see the problem.