美夢成真教甄討論區

393/4 - n ＞ 0
n ≦ 98

19/2 - √(393/4 - n) ≧ 0
n ≧ 8

令 x = 19/2 + √(393/4 - n)，y = 19/2 - √(393/4 - n)
x + y = 19，xy = n - 8

a = √x + √y
a^2 = x + y + 2√(xy) = 19 + 2√(n - 8)

檢驗可知 n = 17，a = 5