a>b>c,比较1/a-b +1/b-c与4/a-c的大小

1/(a-b)+1/(b-c)-4/(a-c)
=(b-c+a-b)/(a-b)(b-c)-4/(a-c)
=(a-c)/(a-b)(b-c)-4/(a-c)
=(a-c)²-4(a-b)(b-c) /(a-b)(b-c)(a-c)
=a²-2ac+c²-4ab+4ac+4b²-4bc /(a-b)(b-c)(a-c)
=a²