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See the diagram attached below.
Let the chords be AB and AC with common point A.
AD is the diameter. Join B with D and C with D to form two triangles.
We need to prove that AB=AC.

In\ \triangle ABD\ and \triangle ACD;\\Given\ that\ \angle BAD=\angle CAD----(condition\ 1)\\since\ AD\ is\ diameter, \angle ABD=\angle ACD = 90^0\\So\ \angle ADB=\angle ADC--------(condition\ 2)\\AD=AD\ (common\ side)-----(condition\ 3)\\ \\So\ the\ triangles\ are\ congruent\ by\ ASA\ rule.\\Hence\ AB=AC.