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Maths › Geometry › Area ›

free triangle

Computes the area of a triangle or a trapezium within a triangle with a fixed vertex.

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Contents

Interface
Free Triangle
Page Comments

Private project under development, to help contact the author:

Interface

C++

Free Triangle

doublefree_triangle(	double	`a`
	double	`b`
	double	`c`
	double	`h`
	double	`alpha`	)[inline]

This module computes the area of the triangle or trapezium formed between a triangle with a fixed vertex on a reference line and a line found at a given distance distance from this reference line.

The various cases are described by the images given in the following documentation, where the area which we want to compute is that of the filled shape.

Solution

Let $\inline \mathrm{xOy}$ be an orthogonal coordinate system and let $\inline \triangle ABC$ be an arbitrary triangle so that $\inline A \in \mathrm{Ox}$ and

$BC = a \qquad AC = b \qquad AB = c$

where $\inline a, b, c \in \mathbb{R}_+^*$ are fixed numbers.

Consider $\inline \alpha = \angle CAX$ and $\inline d \parallel \mathrm{Ox}$ so that the distance between the line d and $\inline \mathrm{Ox}$ equals $\inline h \in \mathbb{R}_+$ . Also let $\inline d \cap AB = \{B_1\}$ and $\inline d \cap AC = \{C_1\}$ .

We will first find the lengths of the projections of B and C on $\inline \mathrm{Ox}$ because we can separate different cases based on these values. Let $\inline d(B,\mathrm{Ox}) = h_b$ and $\inline d(C,\mathrm{Ox}) = h_c$ . Since these values are independent of the relations set between them and h we can consider, without loss of generality, the following case: $\inline h_b > h$ and $\inline h_c > h$ .

MISSING IMAGE!

1/free_triangle01-746.jpg cannot be found in /users/1/free_triangle01-746.jpg. Please contact the submission author.

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Let S be the area which we must determine. Based on the values of $\inline h_b$ , $\inline h_c$ and $\inline h$ , we find the following solutions:

1) $\inline h_b \leq h$ , $\inline h_c \leq h \Rightarrow S = \mathcal{A}_{\triangle ABC}$

MISSING IMAGE!

1/free_triangle_2-746.jpg cannot be found in /users/1/free_triangle_2-746.jpg. Please contact the submission author.

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2) $\inline h_b \geq h$ , $\inline h_c \geq h \Rightarrow S = \mathcal{A}_{\triangle AB_1C_1}$

MISSING IMAGE!

1/free_triangle_3-746.jpg cannot be found in /users/1/free_triangle_3-746.jpg. Please contact the submission author.

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3) $\inline h_b < h$ , $\inline h_c > h\f \Rightarrow S = \mathcal{A}_{[ABB_1C_1]}$

MISSING IMAGE!

1/free_triangle_4-746.jpg cannot be found in /users/1/free_triangle_4-746.jpg. Please contact the submission author.

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4) $\inline h_b > h$ , $\inline h_c < h\f \Rightarrow S = \mathcal{A}_{[ABB_1C_1]}$

MISSING IMAGE!

1/free_triangle_5-746.jpg cannot be found in /users/1/free_triangle_5-746.jpg. Please contact the submission author.

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Output:

a = 2.613
b = 1.878
c = 1.209
alpha = 0.61086
 
h = 0.0   Area = 0.00
h = 0.3   Area = 0.14
h = 0.6   Area = 0.56
h = 0.9   Area = 0.97
h = 1.2   Area = 1.04
h = 1.5   Area = 1.04

Example 1

#include <codecogs/maths/geometry/area/free_triangle.h>
#include <stdio.h>
 
int main()
{
  // the lengths of the sides
  double a = 2.613, b = 1.878, c = 1.209;
 
  // the value of the alpha angle in radians
  double alpha = 0.61086;
 
  // display the lenghts of the sides and the alpha angle
  printf("a = %.3lf\nb = %.3lf\nc = %.3lf\nalpha = %.5lf \n\n", a, b, c, alpha);
 
  // display the area for different values of h
  for (double h = 0.0; h < 1.6; h += 0.3)
  printf("h = %.1lf   Area = %.2lf\n", h, 
  Geometry::Area::free_triangle(a, b, c, h, alpha));
 
  return 0;
}

Note

The vertices must be denoted such that the abscissa of $\inline B$ is less than that of $\inline C$ , i.e. as in the given examples.

Parameters

a	first side of the triangle (BC)
b	second side of the triangle (AC)
c	third side of the triangle (AB)
h	the distance between line $\inline d$ and $\inline \mathrm{Ox}$
alpha	the input angle in radians

Returns

The value of the desired area.

Authors

Eduard Bentea (September 2006)

Source Code

This module is private, for owner's use only.

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