Suppose that there exist several representations $(i=$ $1, \ldots, N)$ of Higgs scalars whose charge-zero members acquire vacuum expectation values $v_i$. Show that
$$
\rho=\left(\frac{M_W}{M_Z \cos \theta_W}\right)^2=\frac{\sum v_i^2\left[T_i\left(T_i+1\right)-\frac{1}{4} Y_i^2\right]}{\sum \frac{1}{2} v_i^2 Y_i^2},
$$
where $T_i$ and $Y_i$ are, respectively, the weak isospin and hypercharge of representation $i$. Show that $\rho=1$ if only Higgs doublets with $Y_i=+1$ exist.