SAS (side, angle side) congruents we could use ASA (angle, side angle) any of those to show that triangle ABD is congruent to triangle CBD. and what that does for us. and we could use as I said (Angle,Side,Angle) or (Side,Angle,Side) whatever we like to use for this. what that does for us is that it tells us that the corresponding sides of these triangles are going to be equal. in particular... in particular, the length AD is going to be equal to CD these are corresponding sides. so these are going to be equal to each other. and if we know that they're equal to each other. that add up to x, remember this was an equilateral triangle of length x. we know that this side right over here is going to be x/2... we know this is going to be x/2 not only do we know that, but also knew when we dropped this altitude we showed that this angle has to be congruent to that angle, and their measures have to add up to 60.