You can put this solution on YOUR website!
You didn't tell me what the letters in the formula
for A, besides the letter A would have to represent.
But whatever, it's a matter of breaking it into n congruent
isosceles triangles. Like this regular octagon:
If you had a regular octagon-shaped pie,
you would cut it into 8 congruent triangular
slices like this:
Then you'd take just one piece of the pie,
and find the area of it, and then multiply it
by the number of sides, n. In the case of this regular
octagon, you would multiply the area of that triangle
by 8, but in general you would multiply that area by n.