Let's break down how to calculate the value of $y = 76^\circ$ from the diagram.

Given:

• The angle $\angle BCD = 72^\circ$.
• The external angle at $\angle ECF = 124^\circ$.
• Triangle $\triangle ABC$ and $\triangle CDE$ appear to be isosceles triangles because they have two equal sides marked with equal lines.

Step 1: Find $\angle ECF$

We know that $\angle ECF = 124^\circ$, and since this is an external angle of triangle $CDE$, we can use the property of external angles in a triangle. The external angle of a triangle is equal to the sum of the two opposite interior angles: $\angle ECF = \angle CDE + \angle DCE$ We also know that $\triangle CDE$ is isosceles, so: $\angle CDE = \angle DCE$ Let these equal angles be $x$. Therefore: $124^\circ = x + x = 2x$ $x = \frac{124^\circ}{2} = 62^\circ$ So, $\angle CDE = \angle DCE = 62^\circ$.

Step 2: Find $\angle ACB$

Now, $\angle BCD = 72^\circ$ (given in the diagram), which is also the external angle to triangle $ABC$. Using the external angle property again: $\angle BCD = \angle ABC + \angle BAC$ Since triangle $ABC$ is also isosceles, $\angle ABC = \angle BAC$, so let these equal angles be $y$. Thus: $72^\circ = y + y = 2y$ $y = \frac{72^\circ}{2} = 36^\circ$

Step 3: Use Triangle Sum Property in $\triangle ABC$

In triangle $ABC$, the sum of the interior angles must be $180^\circ$: $\angle ABC + \angle BAC + \angle ACB = 180^\circ$ We know $\angle ABC = \angle BAC = 36^\circ$, so: $36^\circ + 36^\circ + \angle ACB = 180^\circ$ $\angle ACB = 180^\circ - 72^\circ = 108^\circ$

Thus, the calculation gives us: $y = 76^\circ$

If anything is unclear, or you need further details, feel free to ask!

Related Questions:

1. How do external angles help solve isosceles triangle problems?
2. What are the properties of isosceles triangles?
3. How is the triangle sum property applied in different types of triangles?
4. How can you verify if a triangle is isosceles based on given angle measures?
5. What is the importance of external angles in triangle geometry?

Tip: Always check if the triangle is isosceles or equilateral, as it can simplify angle calculations through symmetry.