From The Document Foundation Wiki
- click and drag to draw a triangle of about the size desired;
- select the ▸ menu;
- make sure Keep ratio is unchecked;
- the height so that it equals the width multiplied by 0.866,
- the width so that it equals the height divided by 0.866,or
- both, maintaining the proportion 0.866 between the height and the width;
- If you change the size of the triangle, click Keep ratio to keep the length of the three sides equal.
- Click OK.