Answer:
2(x + √(18² - x²)) = 40
x + √(324 - x²) = 20
√(324 - x²) = 20 - x
324 - x² = 400 - 40x + x²
2x² - 40x + 76 = 0
x² - 20x + 38 = 0
x² - 20x + 100 = 62
(x - 10)² = 62
x - 10 = ±√62
x = 10 ± √62
Area = (10 + √62)(10 - √62)
= 100 - 62 = 38
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