G2.4 In Figure 3, $A B C$ is an isosceles triangle with $\angle A B C=u^{\circ}, A B=B C=a$ and $A C=b$. If the quadratic equation $a x^{2}-\sqrt{2} \cdot b x+a=0$ has two real roots, whose absolute difference is $\sqrt{2}$, find the value of $u$.