The diagonals of an isosceles triangle have equal measure and divide each other into segments of equal measure. So you can construct the diagonals and create two triangles, one side of one being AC and the other corresponding side being BD. Then using the "divide each other" part and the vertical angles theorem you can prove the two triangles congruent by SAS. The AC congruent to BD by CPCTC.
John
My calculator said it, I believe it, that settles it
