因?yàn)?1 + 2 + 3 +4 + 5 + n >=50
n(n+1) / 2 >=50
n^2 + n - 100 > = 0
當(dāng)n=9時(shí) n(n+1) / 2 = 9*10/2 = 45
當(dāng)n=10時(shí) n(n+1) / 2 = 10*11/2 = 55
所以第50項(xiàng)為10