Vector Algebra

The vectors \(3 \mathbf{a}-5 \mathbf{b}\) and \(2 \mathbf{a}+\mathbf{b}\) are mutually perpendicular and the vectors \(a+4 b\) and \(-\mathbf{a}+\mathbf{b}\) are also mutually perpendicular, then the acute angle between \(\mathbf{a}\) and \(\mathbf{b}\) is