to no avail的用法和样例：
例句
- His intervention was of no avail.
他的调停无效。 - His claim to ownership is invalid.
他的所有权是无效的。 - This is a subject into which many scientists have researched with no result.
这是一个虽经很多科学家研究过但尚无结果的课题。 - Seven patients had abnormal urinalysis but no positive finding in culture.
七位病人之尿液常规检查显示白血球增多但培养无结果。
