Please help me write a geometry proof about an isosceles triange!?

Please help me because I'm doing homework and this is the only question I'm having trouble with. I have to write a two column geometry proof. Prove: If an isosceles triangle has an altitude from the vertex to the base, then the altitude bisects the vertex angle. Given: Triangle ABC is isosceles;... show more Please help me because I'm doing homework and this is the only question I'm having trouble with. I have to write a two column geometry proof.

Prove: If an isosceles triangle has an altitude from the vertex to the base, then the altitude bisects the vertex angle.

Given: Triangle ABC is isosceles; Segment CD is the altitude to base segment AB.
To Prove: Segment CD bisects Angle ACB
Plan: ?

Then there are two columns for the proof. One says statements and the other says reasons. There is a picture of the triangle where the base is AB with D in the middle between them. Then the altitude is from D to C and C is on the top. PLEASE help me!
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