I'm really stuck here. I've tried getting the area of the sector/triangle etc. but I'm getting nowhere near that equation. Can anyone please help?
We know the area of the sector is . The area of the triangle is . (do you see why?). So the area of the shaded region is , where is the length of the side of the square.
But we can find in terms of using Law of Cosines. .
The equation should follow with a little bit of algebra.