Green arc : Red arc = 7 : 11

Geometry Level 3

In A B C \triangle ABC , internal angle bisector of B A C \angle BAC divides A B C \triangle ABC 's circumcircle into two arcs whose ratio of lengths is 7 : 11 7:11 . Given that A C B = 36 ° \angle ACB = 36° , find the measure of A B C \angle ABC in degrees.


The answer is 76.

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2 solutions

Jeremy Galvagni
Aug 19, 2018

Since A B M + A C M = 36 0 \stackrel{\frown}{ABM}+\stackrel{\frown}{ACM}=360^{\circ} use the equation 7 x + 11 x = 36 0 7x+11x=360^{\circ} to get x = 2 0 x=20^{\circ} and so A B M = 7 x = 7 2 0 = 14 0 \stackrel{\frown}{ABM}=7x=7\cdot 20^{\circ}=140^{\circ}

A B = 2 × C = 7 2 \stackrel{\frown}{AB} =2\times\angle C = 72^{\circ}

B M = 14 0 7 2 = 6 8 \stackrel{\frown}{BM}=140^{\circ}-72^{\circ}=68^{\circ}

B A M = 1 2 6 8 = 3 4 \angle BAM = \frac{1}{2}\cdot 68^{\circ}=34^{\circ} , so B A C = 6 8 \angle BAC = 68^{\circ}

B = 18 0 = 6 8 3 6 = 76 \angle B= 180^{\circ}=68^{\circ}-36^{\circ}=\boxed{76}^{\circ}

Let O O be the circumcenter of A B C \triangle ABC , and M M be the intersection point of circumcircle and bisector of B A C \angle BAC . Ratio of lengths of minor and major arc of A M AM is 7 : 11 7: 11 , so if R R is the circumradius, we have:

A O M 360 ° 2 π R 360 ° A O M 360 ° 2 π R = A O M 360 ° A O M = 7 11 A O M = 140 ° \dfrac{\dfrac{\angle AOM}{360°}2 \pi R}{\dfrac{360° - \angle AOM}{360°}2 \pi R} = \dfrac{\angle AOM}{360° - \angle AOM} = \dfrac{7}{11} \Rightarrow \angle AOM = 140°

A C M \angle ACM is inscribed, so: A C M = 1 2 A O M = 70 ° \angle ACM = \dfrac{1}{2}\angle AOM = 70° . Also, quadrilateral A B M C ABMC is cyclic, so: B C M = B A M = 1 2 B A C \angle BCM = \angle BAM = \dfrac{1}{2} \angle BAC .

Now we have that A C M = B C A + B C M = 36 ° + 1 2 B A C = 70 ° B A C = 68 ° \angle ACM = \angle BCA + \angle BCM = 36° + \dfrac{1}{2} \angle BAC = 70° \Rightarrow \angle BAC = 68° .

Finally, A B C = 180 ° B A C A C B = 76 ° \angle ABC = 180° - \angle BAC - \angle ACB = \boxed{76°}

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