280 = C(5,2)C(x,2), x>=2.C(x,2) = 280/C(5,2) = 280*(3!*2!)/5! = 280*2/(5*4) = 28 = x(x-1)/2,0 = x^2 - x - 56 = (x-8)(x+7),x=8.[验算 C(5,2)C(8,2) = 5*4*8*7/(2*2) = 5*8*7 = 280]
上课认真点吧
