In the Reducibility chapter of Sipser's Theory of Computation book, an example is:
We reduce A(TM) to HALT(TM). And then we claim that if H decides HALT(TM), then A decides A(TM), but since A(TM) is undecidable, so is HALT(TM)
My question is, if we already know that A(TM) is undecidable, what is the need for deciding HALT(TM). It's a basic question though.
Hope you got it.