Descriptive Geometry/Mathematical Constructions/Area of a Triangle
The area of any right triangle is equal to the summation of the lengths of its sides. Therefore, any right triangle can be unfolded into a straight line whose length equals the triangle's area. By extension, any given triangle can have its area calculated in the same way as long as it is first converted to a right triangle.
To convert a triangle to a right triangle:
1) Choose one side to rotate until it is either vertical or horizontal, and rotate it until it's orthogonal.
2) Draw a line parallel to the now-orthogonal side of the triangle. Make sure it touches the point opposite the orthogonal side.
3) Move the point along the line until a right angle is formed.
4) Rotate the sides of the triangle to form a straight line. This line's length is equal to the area of the starting triangle.
Image for Question 1 on Area of a Triangle. Answer: http://commons.wikimedia.org/wiki/File:Area_of_a_Triangle_Problem_1_answer.pdf
Answer for Question 2 on Area of a Triangle. Answer: http://commons.wikimedia.org/wiki/File:Area_of_a_Triangle_Problem_2_answer.pdf