Solution
= Solution
An <affine function> preserves mixtures, so
$$
U_0(p\lambda+(1-p)\nu)-U_0(p\mu+(1-p)\nu)=p[U_0(\lambda)-U_0(\mu)]>0.
$$
Therefore
$$
\boxed{p\lambda+(1-p)\nu\succ p\mu+(1-p)\nu.}
$$
The positive mixture weight preserves the strict sign; the common component cancels. This is the <independence axiom for lottery preferences> for the <affine utility on probability measures>.