3. In the convex quadrilateral $ABCD$, $\angle ADB=70^{\circ}, \angle CDB=$ $40^{\circ}, \angle BAC=20^{\circ}, \angle BCA=35^{\circ}$. Find the degree measure of the acute angle between $AC$ and $BD$.
(提示: Extend $B D$ to point $P$, such that $D P=D B$. It is only necessary to prove that points $A$, $B$, $C$, and $P$ are concyclic, then $D C=D P=D A, \angle A D C=110^{\circ}$, $\angle A C D=35^{\circ}$. Therefore, the included acute angle is $75^{\circ}$.)