(k²-144/531）/(k²+16/25)
=(k²+16/25-16/25-144/531）/(k²+16/25)
=1-(16/25+144/531）/(k²+16/25)
因此当k^2=0时有最小值
（k²-144/531）/(k²+16/25)的最小值如何算
(k²-144/531）/(k²+16/25)
=(k²+16/25-16/25-144/531）/(k²+16/25)
=1-(16/25+144/531）/(k²+16/25)
因此当k^2=0时有最小值