Solution
= Solution
This is an <asymptotic sequence>. Since
$$
\log(\cos u)\sim-\frac{u^2}{2}\qquad(u\to0),
$$
one has
$$
\frac{\phi_{n+1}(x)}{\phi_n(x)}
\sim x^2\longrightarrow0.
$$
= Solution
This is an <asymptotic sequence>. Since
$$
\log(\cos u)\sim-\frac{u^2}{2}\qquad(u\to0),
$$
one has
$$
\frac{\phi_{n+1}(x)}{\phi_n(x)}
\sim x^2\longrightarrow0.
$$