Vector Algebra

If \(\mathbf{a}=\frac{3}{2} \hat{\mathbf{k}}\) and \(\mathbf{b}=\frac{2 \hat{\mathbf{i}}+2 \hat{\mathbf{j}}-\hat{\mathbf{k}}}{2}\), then angle between \(\mathbf{a}+\mathbf{b}\) and \(\mathbf{a}-\mathbf{b}\) is