An arc is a ___ when its ends are the ends of a diameter.

Answer

To solve the question "An arc is a ___ when its ends are the ends of a diameter," we can follow these steps:

### Step 1: Understand the Definition of an Arc
An arc is a part of the circumference of a circle. It is defined by two endpoints on the circle.

**Hint:** Remember that an arc is always associated with a circle and is determined by its endpoints.

### Step 2: Identify the Role of the Diameter
The diameter of a circle is a straight line that passes through the center of the circle and has its endpoints on the circle. It is the longest chord of the circle.

**Hint:** The diameter divides the circle into two equal halves.

### Step 3: Consider the Endpoints of the Diameter
When the endpoints of the diameter are considered, they represent two points on the circle that are directly opposite each other.

**Hint:** Visualize the circle and mark the endpoints of the diameter.

### Step 4: Visualize the Arc Formed by the Diameter
The arc that connects these two endpoints (the ends of the diameter) will cover half of the circle. This is because the endpoints are at opposite ends of the circle.

**Hint:** Think about how an arc connects two points on a circle and how it would look if it connects the endpoints of a diameter.

### Step 5: Conclude the Type of Arc
Since the arc formed by the endpoints of the diameter covers half of the circle, it is specifically called a semicircle.

**Hint:** Recall that a semicircle is defined as half of a circle.

### Final Answer
An arc is a **semicircle** when its ends are the ends of a diameter.