be unable to do与be not able to do有什么区别

be unable to do与be not able to do有什么区别
be unable to do与be not able to do有什么区别

be unable to do与be not able to do有什么区别
语义没有区别,但感觉“be unable to”表达否定意思比较自然一点.而“be able to”就专门用于肯定意思好了.