Hatcher's ALgebraic Topology contains the following corollary.
A map $f: X \to Y$ between simply-connected $CW$ complexes is a homotopy equivalence if $f_* : H_n(X) \to H_n(Y)$ is an isomorphism for each $n$>
Proof: After replacing $Y$ with the mapping cylinder $M_f$ we take $f$ to be a inclusion $f: X \hookrightarrow Y$. Since $X$ and $Y$ are simply-connected we have that $\pi_1(X,Y)=0$. This relative Hurewicz theorem then says that the first non-trivial homotopy group is isomorphic to the first non-trivial homology group.
He then goes on to claim that $H_n(Y,X)$ is trivial for all $n$. Why is this true??