Solution
= Solution
The <dot product> is $\mathbf a\cdot\mathbf b=-1+2+3=4$, while the <Euclidean norms> are $\sqrt3$ and $\sqrt{14}$. Thus the angle in the stated range is
$$
\boxed{\theta=\arccos\frac4{\sqrt{42}}}.
$$
The positive cosine shows that it is an acute angle.