I am very interested in knot theory, especially in knot groups and knot polynomials. . As is well known, it is easy to calculate the Alexander polynomial from the foundamental group $\pi_{1}(K)$ of a knot $K$ by free calculus.But I now am reading the book of Rolfsen ( knots and links) which get the polynomial by calculating the Alexander invariant from $\pi_{1}(K)$. I want to compute the Alexanderpolynomial of the torus knot $T_{p,q}$ for p and q coprime by the method in Rolfsen's book. There is a hint in his book as following:

(1)The knot group has presentation $G(T_{p,q})=( u,v\mid u^p=v^{-q})$ where $u\mapsto q,v\mapsto p$ under abelianization.

(2)Choose integer $r,s$ satisfying $pr+qs=1,r>0,s<0$.Let $x=u^{s}v^{r},a=ux^{-q},b=vx^{-p}$ to obstain the presentation with $x\mapsto 1,a\mapsto 0,b\mapsto 0$:

$G(T_{p,q})=(x,a,b\mid (ax^{q})^p=(bx^p)^{-q}),x=(ax^q)^s(bx^p)^r)$

(3)Let $ C=[G,G]$ then $C/[C,C]$ has a $\Lambda-$module presentation with generators $\alpha,\beta$ and relations:

$(t^q+t^{2q}+...+t^{pq})\alpha=(t^p+t^{2p}+...+t^{qp})\beta$,

$(t^q+t^{2q}+...+t^{(-s)q})\alpha=(t^p+t^{2p}+...+t^{rp})\beta$

(4)$H_1(\tilde{X})\cong \Lambda/(\Delta(t))$ where

$\Delta(t)=\frac{(t^{pq}-1)(t-1)}{(t^p-1)(t^q-1)}$

I know the (1)-(3),but I do not know how to get the (4).Can someone help me with this?Thanks a lot.