# Largest trapezoid that can be inscribed in a semicircle

Given a semicircle of radius r, the task is to find the largest trapezoid that can be inscribed in the semicircle, with base lying on the diameter.

Examples:

```Input: r = 5
Output: 32.476

Input: r = 8
Output: 83.1384
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution. Approach: Let r be the radius of the semicircle, x be the lower edge of the trapezoid, and y the upper edge, & h be the height of the trapezoid.
Now from the figure,

r^2 = h^2 + (y/2)^2

or, 4r^2 = 4h^2 + y^2

y^2 = 4r^2 – 4h^2

y = 2√(r^2 – h^2)

We know, Area of Trapezoid, A = (x + y)*h/2

So, A = hr + h√(r^2 – h^2)

taking the derivative of this area function with respect to h, (noting that r is a constant since we are given the semicircle of radius r to start with)

dA/dh = r + √(r^2 – h^2) – h^2/√(r^2 – h^2)

To find the critical points we set the derivative equal to zero and solve for h, we get

h = √3/2 * r

So, x = 2 * r & y = r

So, A = (3 * √3 * r^2)/4

Below is the implementation of above approach:

## C++

 `// C++ Program to find the biggest trapezoid ` `// which can be inscribed within the semicircle ` `#include ` `using` `namespace` `std; ` ` `  `// Function to find the area ` `// of the biggest trapezoid ` `float` `trapezoidarea(``float` `r) ` `{ ` ` `  `    ``// the radius cannot be negative ` `    ``if` `(r < 0) ` `        ``return` `-1; ` ` `  `    ``// area of the trapezoid ` `    ``float` `a = (3 * ``sqrt``(3) * ``pow``(r, 2)) / 4; ` ` `  `    ``return` `a; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``float` `r = 5; ` `    ``cout << trapezoidarea(r) << endl; ` `    ``return` `0; ` `} `

## Java

 `// Java Program to find the biggest trapezoid ` `// which can be inscribed within the semicircle ` ` `  `import` `java.util.*; ` `import` `java.lang.*; ` `import` `java.io.*; ` ` `  `class` `GFG{ ` `// Function to find the area ` `// of the biggest trapezoid ` `static` `float` `trapezoidarea(``float` `r) ` `{ ` ` `  `    ``// the radius cannot be negative ` `    ``if` `(r < ``0``) ` `        ``return` `-``1``; ` ` `  `    ``// area of the trapezoid ` `    ``float` `a = (``3` `* (``float``)Math.sqrt(``3``)  ` `            ``* (``float``)Math.pow(r, ``2``)) / ``4``; ` ` `  `    ``return` `a; ` `} ` ` `  `// Driver code ` `public` `static` `void` `main(String args[]) ` `{ ` `    ``float` `r = ``5``; ` `    ``System.out.printf(``"%.3f"``,trapezoidarea(r)); ` `} ` `} `

## Python 3

 `# Python 3 Program to find the biggest trapezoid  ` `# which can be inscribed within the semicircle  ` ` `  `# from math import everything ` `from` `math ``import` `*` ` `  `# Function to find the area  ` `# of the biggest trapezoid  ` `def` `trapezoidarea(r) : ` ` `  `    ``# the radius cannot be negative  ` `    ``if` `r < ``0` `: ` `        ``return` `-``1` ` `  `    ``# area of the trapezoid ` `    ``a ``=` `(``3` `*` `sqrt(``3``) ``*` `pow``(r,``2``)) ``/` `4` ` `  `    ``return` `a ` ` `  ` `  `# Driver code      ` `if` `__name__ ``=``=` `"__main__"` `: ` ` `  `    ``r ``=` `5` ` `  `    ``print``(``round``(trapezoidarea(r),``3``)) ` ` `  ` `  `# This code is contributed by ANKITRAI1 `

## C#

 `// C# Program to find the biggest  ` `// trapezoid which can be inscribed  ` `// within the semicircle ` `using` `System; ` ` `  `class` `GFG ` `{ ` `// Function to find the area ` `// of the biggest trapezoid ` `static` `float` `trapezoidarea(``float` `r) ` `{ ` ` `  `    ``// the radius cannot be negative ` `    ``if` `(r < 0) ` `        ``return` `-1; ` ` `  `    ``// area of the trapezoid ` `    ``float` `a = (3 * (``float``)Math.Sqrt(3) *  ` `                   ``(``float``)Math.Pow(r, 2)) / 4; ` ` `  `    ``return` `a; ` `} ` ` `  `// Driver code ` `public` `static` `void` `Main() ` `{ ` `    ``float` `r = 5; ` `    ``Console.WriteLine(``""` `+ trapezoidarea(r)); ` `} ` `} ` ` `  `// This code is contributed  ` `// by inder_verma `

## PHP

 ` `

Output:

```32.476
```

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