lose interest in & have an interest in

题目

lose interest in & have an interest in
为什么在lose interest in 中,这个interest 作为可数名词不需要加an,而在后面的那个词组当中却要加an.我想请问下,这个到底是为什么啊?什么情况下要,什么情况下不要?还是我看的书是盗版的?本来两个就都有an?

答案

interest=readiness to get attention兴趣时,用作可数或不可数名词都可以,例如He does not have much interest in sport.(摘自英汉多功能词典)he is showing an interest in music.(摘自LONGMAN词典)
所以.你的书没错