How to demonstrate that S = S' x Cosa

[Jadsadaka]

Hey, let s consider two planes, and they make an angle a between them. On one of the planes there is a figure with the surface S. I want to prove that the surface of the same figure projected on the second plane is equal to the surface of the figure on the first plane times the cosine of the angle formed between the two planes.
P.S: 1- Sorry for any English mistakes i ve translated this from french
2- I make no distinguishment between area and surface so consider the right term

 

[Ishuda]

Hey, let s consider two planes, and they make an angle a between them. On one of the planes there is a figure with the surface S. I want to prove that the surface of the same figure projected on the second plane is equal to the surface of the figure on the first plane times the cosine of the angle formed between the two planes.
P.S: 1- Sorry for any English mistakes i ve translated this from french
2- I make no distinguishment between area and surface so consider the right term

Hint: First start with a simple line. What is the length of the line projected on the second plane? If that line were one side of a rectangle, what is the ratio of the two areas?