Indian Statistical Institute (ISI) B.Math & B.Stat : Algebra

If $p,q,r$ are positive real numbers such that $pqr=1$, then find the value of $\frac{1}{1+p+q^{-1}}+\frac{1}{1+q+r^{-1}}+\frac{1}{1+r+p^{-1}}$. Throught the simplification we will use $1=pqr,q^{-1}=pr,r^{-1}=pq \quad and \quad p^{-1} = qr$ Given expression is $$\frac{1}{1+p+q^{-1}}+\frac{1}{1+q+r^{-1}}+\frac{1}{1+r+p^{-1}}$$ $$= \frac{pqr}{pqr+p+pr}+\frac{pqr}{pqr+q+pq}+\frac{1}{1+r+p^{-1}}$$ $$= \frac{qr}{qr+1+r}+\frac{pr}{pr+1+p}+\frac{1}{1+r+p^{-1}}$$ $$= \frac{qr}{p^{-1}+1+r}+\frac{pr}{pr+pqr+p}+\frac{1}{1+r+p^{-1}}$$ $$= \frac{qr}{p^{-1}+1+r}+\frac{r}{r+qr+1}+\frac{1}{1+r+p^{-1}}$$ $$= \frac{qr}{p^{-1}+1+r}+\frac{r}{r+p^{-1}+1}+\frac{1}{1+r+p^{-1}}$$ $$= \frac{qr+r+1}{p^{-1}+1+r}$$ $$= \frac{p^{-1}+r+1}{p^{-1}+1+r}$$ $$= 1$$