The problem in the image appears to be asking for a geometric explanation related to the angles of a quadrilateral $ABCD$. The text translates to:

"There is a quadrilateral $ABCD$ as shown above. When the auxiliary line is drawn between points $A$ and $C$, prove that $\angle A + \angle B + \angle C = \angle D$."

Approach:

1. The quadrilateral $ABCD$ has an internal point $D$, and a diagonal is drawn between $A$ and $C$.
2. We are asked to prove that the sum of the angles at $A$, $B$, and $C$ equals the angle at $D$.

Steps to solve:

1. Angle Sum Property of a Triangle: For any triangle, the sum of its interior angles is always 180°. This is a key geometric property to use here.

2. Consider the triangles formed: The diagonal from $A$ to $C$ splits the quadrilateral into two triangles:

   • Triangle $ABD$
   • Triangle $BCD$

   In each triangle, the sum of the angles will be 180°.

3. Angle Analysis:

   • In $\triangle ABD$, $\angle A + \angle B + \angle D_1 = 180^\circ$.
   • In $\triangle BCD$, $\angle C + \angle B + \angle D_2 = 180^\circ$.

   Here, $\angle D_1$ and $\angle D_2$ are parts of the same angle $\angle D$.

4. Combining angles: By summing the interior angles of the two triangles, we can show that: $\angle A + \angle B + \angle C = \angle D$ because $\angle D_1 + \angle D_2 = \angle D$.

This provides the geometric justification for the relationship between the angles. Would you like more details or further clarification on any part of the solution?

Relative Questions:

1. How do the angle sum properties apply to different polygons?
2. What is the significance of using auxiliary lines in geometry?
3. How can we generalize this result to any quadrilateral?
4. What is the role of internal points in geometric constructions?
5. How do we prove angle relations using parallel lines?

Tip: In geometry, auxiliary lines can greatly simplify problems by breaking down complex shapes into simpler components like triangles.