SOLIDWORKS has hidden functions for some tools. This is the case with command Trim elements, which also allows for extension, even though there is a separate operation for lengthening.

But today I wanted to show, useful option. Well, in the mode power trimming, if you want to extend a sketch segment Until the next one – cut it with Shift!

However, if you grab the line traditionally, you can extend it manually, and then holding will point to another line – the program will extend the former to extend the latter.