Prove: It is easy to prove with what is known.
△ODC is all equal to△△ OAB (SAS)
So ∠C=∠A
So DC//AB (internal dislocation angles are equal and two straight lines are parallel)
2. In △ABC, AB=AC, point D is the midpoint of BC, and point E is on AD. Find the triangles in the diagram and explain why they are congruent.
Triangle ABD≌ triangle ACD
Because point D is the midpoint of BC, BD=CD.
And because AB=AC and AD are common sides, according to SSS, two triangles are congruent.
Triangle ABE?/ triangle ACE
Because point e is on the center line of triangle ABC,
So BE=CE
And because AB=AC, AE is the common side, according to SSS, two triangles are congruent.
Triangle EBD≌ triangle ECD
Because BD=CD BE=CE ED is the same side, according to SSS, two triangles are congruent.
Look at the answer.
The answer is that the cosine of the vector PA and the normal vector N =√2/4, and the normal vector