P is a point in triangle A B C such that ∠ P A B = ∠ P A C = 2 2 ∘ , ∠ P B A = ∠ P B C = 3 3 ∘ . What is the measure (in degrees) of ∠ A P C ?
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How would you show that the 3 angle bisectors are congruent?
P is the incircle of this triangle. So, C P must be the angle bisector of ∠ C .
Note that P is the incenter of △ A B C . (since it is the intersection of the two angle bisectors.) Thus, CP is the angle bisector of ∠ A C B . So, ∠ A C P = 3 5 ∘ . Thus, ∠ A P C = 1 2 3 ∘ .
Since the sum of the angles of a triangles are 180, we have that ∠ B C A = 1 8 0 − 4 4 − 3 3 = 7 0 . We also know that the three angle bisectors are concurrent, which means that since P is on two of them, it must also be on the third and ∠ P C A = 2 7 0 = 3 5 . Finally since the sum of the angles of △ A P C is 180, we can find that ∠ A P C = 1 8 0 − 2 2 − 3 5 = 1 2 3 ∘
How would you show that "the three angle bisectors are concurrent"?
33** 66
Nobody's answered and it's been a few days so I'll chip in...
Let O be the point such that O A bisects ∠ A and O B bisects ∠ B . Drop perpendiculars from O onto each side of the triangle, intersecting A B at D , B C at E , and C A at F . Then △ O A D and △ O A F are congruent because their angles are identical and they share the side O A . Similarly △ O B D and △ O B E are congruent, so the three perpendiculars are all the same length. This means that △ O C E and △ O C F must be congruent since they share O C , O E = O F and the right angle. So O C bisects C .
From the given information, we can deduce that P is on the angle bisector of ∠ A B C and on the angle bisector of ∠ B A C . We know that the only point for which this happens is the incenter, and so P is also on the angle bisector of ∠ A C B .
We can very easily find that m ∠ A C B = 7 0 ∘ , which means that m ∠ A C P = 3 5 ∘ . We use this and the fact that m ∠ P A C = 2 2 ∘ to compute the desired value, m ∠ A P C .
m ∠ A P C = 1 8 0 ∘ − 2 2 ∘ − 3 5 ∘ = 1 2 3 ∘ .
Since two angle bisectors determine a unique incenter, P is the incenter of triangle A B C . ∠ P C A = 2 1 ( 1 8 0 ∘ − 6 6 ∘ − 4 4 ∘ ) = 3 5 ∘ . ∠ A P C = 1 8 0 ∘ − 2 2 ∘ − 3 5 ∘ = 1 2 3 ∘ .
(180-2(22)-2(33))/2=35
180-22-35=123
Make line AP and BP. Since AP bisect ∠ B A C and BP bisect ∠ A B C then AP and BP are angle bisectors, and we deduce that P is incenter of triangle ABC, So, CP is angle bisector too. We can easily get ∠ A C B = 7 0 , ∠ A C P = 3 5 and ∠ A P C = 1 2 3
Since ∠ P A B = ∠ P A C and ∠ P B A = ∠ P B C , P is the incenter of A B C . Thus, ∠ P C A = ∠ P C B = 2 1 8 0 ∘ − 2 ( 2 2 ∘ + 3 3 ∘ ) = 3 5 ∘ . Therefore, ∠ A P C = 1 8 0 ∘ − ∠ P A C − ∠ P C A = 1 8 0 ∘ − 2 2 ∘ − 3 5 ∘ = 1 2 3 ∘ .
Remark. We can find ∠ A P C by using the formula ∠ A P C = 9 0 ∘ + 2 A B C = 9 0 ∘ + ∠ P B C = 1 2 3 ∘ . Note that this formula is obtained by similar arguments as above.
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A P is the angle bisector of ∠ A , B P is the angle bisector of ∠ B . And A P and B P meets at P . So C P is also the angle bisector of ∠ C
Now, ∠ C = 1 8 0 ∘ − ∠ A − ∠ B
⇒ ∠ C = 7 0
⇒ ∠ P C A = 3 5
⇒ ∠ A P C = 1 8 0 ∘ − 2 2 ∘ − 3 5 ∘
⇒ ∠ A P C = 1 2 3