[คusักคณิm]

abc+ab+bc+ca+a+b+c=1000
abc+ab+ac+a+bc+b+c+1=1001
ab(c+1)+a(c+1)+b(c+1)+c+1=1001
(ab+a+b+1)(c+1)=7x11x13
(a+1)(b+1)(c+1)=7x11x13
แสดงว่า a+b+c=6+10+12=28