Angle bisectors

Geometry Level 3

P P is a point in triangle A B C ABC such that P A B = P A C = 2 2 \angle PAB = \angle PAC = 22 ^\circ , P B A = P B C = 3 3 \angle PBA = \angle PBC = 33 ^\circ . What is the measure (in degrees) of A P C \angle APC ?


The answer is 123.

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8 solutions

Kiriti Mukherjee
Aug 4, 2013

A P AP is the angle bisector of A \angle A , B P BP is the angle bisector of B \angle B . And A P AP and B P BP meets at P P . So C P CP is also the angle bisector of C \angle C

Now, C = 18 0 A B \angle C = 180^\circ - \angle A - \angle B

C = 70 \Rightarrow \angle C = 70

P C A = 35 \Rightarrow \angle PCA = 35

A P C = 18 0 2 2 3 5 \Rightarrow \angle APC = 180^\circ - 22^\circ - 35^\circ

A P C = 123 \Rightarrow \angle APC = \boxed {\boxed {123}}

Moderator note:

How would you show that the 3 angle bisectors are congruent?

P P is the incircle of this triangle. So, C P CP must be the angle bisector of C \angle C .

Kiriti Mukherjee - 7 years, 10 months ago

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Incentre* not incircle.

Krishna Ar - 6 years, 8 months ago
Zi Song Yeoh
Aug 5, 2013

Note that P is the incenter of A B C \triangle ABC . (since it is the intersection of the two angle bisectors.) Thus, CP is the angle bisector of A C B \angle ACB . So, A C P = 3 5 \angle ACP = 35^{\circ} . Thus, A P C = 12 3 \angle APC = 123^{\circ} .

Francisco Rivera
Aug 4, 2013

Since the sum of the angles of a triangles are 180, we have that B C A = 180 44 33 = 70 \angle BCA = 180 - 44 - 33 = 70 . We also know that the three angle bisectors are concurrent, which means that since P P is on two of them, it must also be on the third and P C A = 70 2 = 35 \angle PCA = \dfrac{70}{2} = 35 . Finally since the sum of the angles of A P C \triangle APC is 180, we can find that A P C = 180 22 35 = 12 3 \angle APC = 180 - 22 - 35 = \boxed{123^\circ}

Moderator note:

How would you show that "the three angle bisectors are concurrent"?

33** 66

Karan Jain - 7 years, 10 months ago

Nobody's answered and it's been a few days so I'll chip in...

Let O O be the point such that O A OA bisects A \angle A and O B OB bisects B \angle B . Drop perpendiculars from O O onto each side of the triangle, intersecting A B AB at D D , B C BC at E E , and C A CA at F F . Then O A D \triangle OAD and O A F \triangle OAF are congruent because their angles are identical and they share the side O A OA . Similarly O B D \triangle OBD and O B E \triangle OBE are congruent, so the three perpendiculars are all the same length. This means that O C E \triangle OCE and O C F \triangle OCF must be congruent since they share O C OC , O E = O F OE = OF and the right angle. So O C OC bisects C C .

Matt McNabb - 7 years, 10 months ago

From the given information, we can deduce that P P is on the angle bisector of A B C \angle ABC and on the angle bisector of B A C \angle BAC . We know that the only point for which this happens is the incenter, and so P P is also on the angle bisector of A C B \angle ACB .

We can very easily find that m A C B = 7 0 m \angle ACB = 70^\circ , which means that m A C P = 3 5 m \angle ACP = 35^\circ . We use this and the fact that m P A C = 2 2 m \angle PAC = 22^\circ to compute the desired value, m A P C m \angle APC .

m A P C = 18 0 2 2 3 5 = 123 m \angle APC = 180^\circ - 22^\circ - 35^\circ = \fbox{123}^\circ .

Qi Huan Tan
Aug 10, 2013

Since two angle bisectors determine a unique incenter, P P is the incenter of triangle A B C ABC . P C A = 1 2 ( 18 0 6 6 4 4 ) = 3 5 \angle PCA=\frac{1}{2}(180^\circ-66^\circ-44^\circ)=35^\circ . A P C = 18 0 2 2 3 5 = 12 3 \angle APC=180^\circ-22^\circ-35^\circ=123^\circ .

Yong Daniel
Aug 5, 2013

(180-2(22)-2(33))/2=35

180-22-35=123

Rizky Dermawan
Aug 4, 2013

Make line AP and BP. Since AP bisect B A C \angle BAC and BP bisect A B C \angle ABC 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 = 70 \angle ACB = 70 , A C P = 35 \angle ACP = 35 and A P C = 123 \angle APC = 123

Duc Minh Phan
Aug 4, 2013

Since P A B = P A C \angle PAB = \angle PAC and P B A = P B C \angle PBA = \angle PBC , P P is the incenter of A B C ABC . Thus, P C A = P C B = 18 0 2 ( 2 2 + 3 3 ) 2 = 3 5 \angle PCA = \angle PCB = \frac{180^\circ-2(22^\circ+33^\circ)}{2}=35^ \circ . Therefore, A P C = 18 0 P A C P C A = 18 0 2 2 3 5 = 12 3 \angle APC = 180^\circ - \angle PAC - \angle PCA = 180^\circ-22^\circ-35^\circ = 123^\circ .

Remark. We can find A P C \angle APC by using the formula A P C = 9 0 + A B C 2 = 9 0 + P B C = 12 3 \angle APC = 90^\circ + \frac{ABC}{2} = 90^\circ + \angle PBC = 123^\circ . Note that this formula is obtained by similar arguments as above.

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