(1/(a+b+c)+1/(b+c+d)+1/(c+d+a)+1/(d+a+b))*((a+b+c)+(b+c+d)+(c+d+a)+(d+a+b))>16所以1/(a+b+c)+1/(b+c+d)+1/(c+d+a)+1/(d+a+b)>16/3(a+b+c+d)(利用柯西不等式)
