This section of the larger tutorial trail is limited to a few observations concerning using UseCases in SysML, it is not a full introduction to UseCase diagrams.
Slides in this section
If you want to navigate the entire section including the additional explanatory text click on the title link of the first slide to view the full slide page, then use the next links to move through the slide pages. If you just want to view the slide images only in sequence, click on any slide to view it larger in a viewer, then click again to move through each slide in this tutorial trail section.
There are some more examples of UseCase Diagrams in the Hybrid SUV sample problem tutorial trail.