Calculating perimeter by area formula

[shahar]

can be a way to calculate area of a polygon by the formula of area of:

?

 

[Dr.Peterson]

Are you asking about area, perimeter, or both???

What do a, b, and gamma mean in that formula? It looks like a formula for the area of a triangle, not a general polygon.

Are you talking about a regular polygon?

 

[shahar]

I meant triangle.
How can I use other formula from the formula above to calculate perimeter?

 

[Dr.Peterson]

Area is not directly related to perimeter; you can't get one from the other. (As a simple example, there are many rectangles with a given area, which have different perimeters.) Why do you think you could use the area formula to find the perimeter?

Apparently you know two sides and the included angle, and want to find the perimeter of the triangle. All you need is the other side, right? Do you know the Law of Cosines?

 

[shahar]

The law of Cosines:
I learn from the Hebrew Wikipedia that it was define in the "Element" of Euclid, firstly, by Geometric proof.
It is: a*b*cos(the Cosinus between the angle that its vertex is the common vertex of a & b).
Thanks.