What is the best definition of a slant? someone's opinion that is unfounded an author's bias for or against something an inappropriate source of information that cannot be used in research an angled or inclined surface

The rise of advertising in the 1920s. i am good with seeing how advertisements shape the roaring twenties and how it developed a new way of life through modern days. but i am not sure what the details of advertising in the 1920s were. if possible can you send me links (preferrably credential links) to sources that maybe show like the first company to start advertising through the radio, how advertising benefits (statistics graphs), etc.