I've only seen crochet patterns for half squares which don't look like a complete triangle ...
... so I experimented with patterns of my own. So far I've come up with 2 ways, one a little more complicated than the other. Here's the first one I tried. As you can see the clusters are smaller along the hypotenuse.
Here's my second attempt. This one is easier to make and the clusters are all trebles as per a normal Granny Square. I've not tried a larger version yet.
The reason I wanted a Right Angled Granny Triangle is (I'm sad) because I wanted the layout of my 2012 blanket to be a slightly crazy one with odd shapes and squares going here and there at different angles.
Here's one way of fitting in my Granny Rhombi. I may make them to zig-zag round the edge of the blanket interspersed with equilateral triangles.
Here endeth the Mathematics lesson on trigonometry, shape and tessellating. :)