360° ÷ 10 ÷ 2 = 18°
So the length of the decagon side is:
10 × tan18° × 2 = 20 × tan18°
The area is: ½ × 20 × tan18° × 10 × 10 = 1000 × tan18°
≈ 324.9
.: 324.9 is the final answer.
I hope this helps.
