Given: Triangle ABC is isosceles; 
       Line segment CD is the altitude to the base AB

To prove: Angle ACD = angle BCD




angle A = angle B       Base angles of isosceles triangle

angle ADC = 90°         An altitude of a triangle is 
                        perpendicular to a side

angle BDC = 90°         An altitude of a triangle is 
                        perpendicular to a side

angle ADC = angle BDC   Both are right angles 

Angle ACD = angle BCD   If two angles of one triangle are
                        equal to two angles of another
                        triangle, the third angles are 
                        equal.

 Edwin