Prove every segment has a midpoint.
Unfortunately I do not have the definition yet of isosceles triangles. All I have is SSS and SAS. I also do not have right angles. But I do have perpendicular lines and bisectors.
If we are in euclidean geometry you forget the circle,infact if we have a segment AB,
let's take the circle C1 with center in A and radius the lenght of AB and the circle C2
with center in B and radius AB,if we intersect these 2 circle we obtain two different points,
let's call them P and Q,they have the same distance from the A and B,if we call r the straight line
for P and Q,r is the AXIS of the segment,in short words the locus of the points with the same distance from A and B,and the intersection between r and the segment AB is the middle point.
The straight r is perpendicular to AB then this intersection always exists.
Here it is an animation that explain the construction.