Construct the lines AX, BX, and CX.
First, let’s begin with what we know to be true:
Because M is the midpoint of AB, AM = MB.
Because MX is the perpendicular bisector of AB, angle AMX = angle BMX = 90 deg.
Therefore AMX is congruent to BMX.
This means that all the angles and side of these two triangles are equal.
Therefore AX = BX.
By a similar proof, it can be seen that BX = CX
Since AX = BX = CX, then AX = CX.