# Indirect proof

• Feb 15th 2011, 05:27 PM
jam2011
Indirect proof
PROVE THAT the bisector of an angle of a scalene triangle cannot be perpendicular to the opposite side..

Can any one give the explanation or discussion about this problem since i was absent that time when this topic is being discuss. Please give me hint how to do this ? thanks
• Feb 15th 2011, 05:38 PM
topsquark
Quote:

Originally Posted by jam2011
PROVE THAT the bisector of an angle of a scalene triangle cannot be perpendicular to the opposite side..

Can any one give the explanation or discussion about this problem since i was absent that time when this topic is being discuss. Please give me hint how to do this ? thanks

Perhaps you could start here: consider an isosceles triangle and a line bisecting the vertex angle. Is this line perpendicular to the base?

-Dan
• Feb 18th 2011, 02:44 AM
Kaloda
Proof:

Let ABC be a scalene triangle and D be the foot of the bisector of angle A at BC. Suppose that BC is perpendicular to AD, then it follows that angle DAB and angle ADB are congruent to angle DAC and angle ADC, respectively. Since AD=AD by reflexive property, then triangle ABD is congruent to triangle ACD by ASA, and thus AC = AB by CPCTC. But by defn. of Scalene triangle, AC cannot be congruent to AB and therefore a contradiction follows and the supposition is false and the theorem is true!
• Feb 25th 2011, 05:45 AM
jam2011
, then it follows that angle DAB and angle ADB are congruent to angle DAC and angle ADC, respectively.

THEN WHAT REASON THUS :angle DAB and angle ADB are congruent to angle DAC and angle ADC..OR WHAT DEFINITION......