how do we know that the area of the triangle above is the same as a congruent triangle in the nonagon? What if you used the Pythagoras theorem to find the height of the triangle and then multiplied 1/2hb to get the area?
How can you find the height of the triangle?
Draw a perpendicular line down the middle of this triangle and use the Pythagoras theorem to find height
OK, and how do you do that without trignomentry?