介词加关系代词在定语从句中做什么成分?

1.做定语,如：The book in which the pictures are black and white is mine.
主句是：The book is mine.
定从是：The pictures are black and white.
在复合句中in chich作定语.
2.作状语,如：The boy with whom I will go to your home is my best friend.
主句是：The boy is my best friend.
定从是：I will go to your home with him.
在复合句中with whom作状语.

这是把介词提前，后接which，which做介词宾语，整个介词短语做定语从句的状语，有的时候介词短语等于where