To prove that the triangle of maximum area inscribed in a given circle is equilateral, we will use geometry and calculus. Let’s break it down step by step.

Step 1: General Properties of the Inscribed Triangle

• Let the given circle have a radius $R$ and center $O$.
• Consider an arbitrary triangle $\triangle ABC$ inscribed in the circle. This means that all three vertices $A, B, C$ lie on the circle.
• The area of a triangle inscribed in a circle is maximized when the triangle is equilateral, but we need to formally prove this.

Step 2: Formula for the Area of a Triangle in Terms of the Circumradius

For any triangle inscribed in a circle, the area $A$ of the triangle can be expressed as:

$A = \frac{1}{2} \cdot a \cdot b \cdot \sin(\theta)$

where:

• $a$ and $b$ are two sides of the triangle.
• $\theta$ is the angle between these two sides.

Using the fact that the sides of the triangle can also be related to the circle's radius, we utilize another important formula for the area of a triangle inscribed in a circle:

$A = \frac{1}{2} \cdot R^2 \cdot \sin(\alpha) \cdot \sin(\beta) \cdot \sin(\gamma)$

where $\alpha, \beta, \gamma$ are the angles of the triangle, and $R$ is the circumradius.

Step 3: Symmetry and Equal Angles

For the area of the triangle to be maximized, the symmetry of the angles plays a crucial role. To maximize the area, it is intuitive that the triangle should have maximum symmetry, meaning:

$\alpha = \beta = \gamma = \frac{\pi}{3}$

This gives an equilateral triangle, where all sides and angles are equal.

Step 4: Maximizing the Area

Let’s calculate the area when the triangle is equilateral. For an equilateral triangle inscribed in a circle:

• All angles are $\frac{\pi}{3}$.
• The area formula simplifies since $\sin\left(\frac{\pi}{3}\right) = \frac{\sqrt{3}}{2}$.

So the area of the equilateral triangle becomes:

$A_{\text{max}} = \frac{3\sqrt{3}}{4} \cdot R^2$

This is the maximum possible area for any triangle inscribed in the circle.

Step 5: Conclusion

Thus, the triangle of maximum area inscribed in a given circle is equilateral. This is because the symmetry of the equilateral triangle maximizes the area compared to any other configuration, as it equally distributes the sides and angles, which optimizes the sine function for each angle.

Would you like more details or clarifications on any step?

Here are some related questions to consider:

1. How do we compute the area of any triangle given the radius and angles?
2. What are the properties of triangles inscribed in circles?
3. Can a triangle with unequal angles ever have a larger area than an equilateral one?
4. How does this result apply to triangles in ellipses instead of circles?
5. What happens if the circle is not a unit circle, and how do different radii affect the area?

Tip:

When optimizing geometric shapes, symmetry often plays a key role in maximizing or minimizing properties like area or perimeter.