1. A method of identifying topics in lecture videos, the method comprising: receiving lecture video metadata, learning courses metadata, and a lecture video transcript having transcribed text of a lecture video; discovering one or more candidate learning courses that are related to the lecture video based on a measured similarity between the lecture video metadata and the learning courses metadata; extracting key phrases from learning materials of the one or more candidate learning courses; assigning weights to the extracted key phrases based on a position of the extracted key phrases in the learning materials, frequencies with which the extracted key phrases appear in the learning materials, and the discovered candidate learning course in which the key phrases appear; and apportioning the lecture video into two or more topic-specific portions based on two or more topic segments generated in the lecture video transcript, presence of the extracted key phrases within the two or more topic segments, and the assigned weights of the extracted key phrases.