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Geometry Level 3

Triangle A B C ABC is isosceles with A C = B C AC = BC and A C B = 10 6 \angle ACB = 106^\circ Point M M is inside the triangle so that M A C = 7 \angle MAC = 7^\circ and M C A = 2 3 \angle MCA = 23^\circ . Find the measure of C M B \angle CMB in degrees.


The answer is 83.

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3 solutions

Chew-Seong Cheong
May 10, 2015

Let C M B = θ \angle CMB = \theta . Using Sine Rule, we have:

A C M : C M sin M A C = A C sin C M A C M sin 7 = A C sin C M A C M = A C sin 7 sin 15 0 = A C sin 7 sin 3 0 = 2 A C sin 7 B C M : C M sin M B C = B C sin C M B C M sin ( 9 7 θ ) = B C sin θ C M = B C cos ( θ 7 ) sin θ = A C cos ( θ 7 ) sin θ \def\arraystretch{2.5} \begin{array}{c}\triangle ACM: & \dfrac {CM}{\sin{\angle MAC}} = \dfrac{AC}{\sin{\angle CMA}} \quad \Rightarrow \dfrac {CM}{\sin{7^\circ}} = \dfrac{AC}{\sin{\angle CMA}} \\ & \Rightarrow CM = \dfrac {AC\sin{7^\circ}}{\sin{150^\circ}} = \dfrac {AC\sin{7^\circ}}{\sin{30^\circ}} = 2AC\sin{7^\circ} \\ \triangle BCM: & \dfrac {CM}{\sin{\angle MBC}} = \dfrac{BC}{\sin{\angle CMB}} \quad \Rightarrow \dfrac {CM}{\sin{(97^\circ-\theta)}} = \dfrac{BC}{\sin{\theta}} \\ & \Rightarrow CM = \dfrac {\color{#3D99F6}{BC}\cos{(\theta-7^\circ)}}{\sin{\theta}} = \dfrac {\color{#3D99F6}{AC}\cos{(\theta-7^\circ)}}{\sin{\theta}} \end{array}

2 sin 7 = cos ( θ 7 ) sin θ 2 sin 7 sin θ = cos ( θ 7 ) 2 sin 7 sin θ = cos 7 cos θ + sin 7 sin θ cos 7 cos θ sin 7 sin θ = 0 cos θ + 7 = 0 θ + 7 = 9 0 C M B = θ = 9 0 7 = 8 3 \begin{aligned} \Rightarrow 2\sin{7^\circ} & = \frac {\cos{(\theta-7^\circ)}}{\sin{\theta}} \\ 2\sin{7^\circ} \sin{\theta} & = \cos{(\theta-7^\circ)} \\ 2\sin{7^\circ} \sin{\theta} & = \cos{7^\circ} \cos{\theta} + \sin{7^\circ} \sin{\theta} \\ \cos{7^\circ} \cos{\theta} - \sin{7^\circ} \sin{\theta} & = 0 \\ \cos{\theta + 7^\circ} & = 0 \\ \Rightarrow \theta + 7^\circ & = 90^\circ \\ \Rightarrow \angle CMB & = \theta = 90^\circ-7^\circ = \boxed{83^\circ} \end{aligned}

Klaus Kübbeler
Feb 6, 2017

채범 신
Nov 24, 2015

Let Triangle BCD be an equilateral triangle. Following from AC=CB=CD, the angle of CAD=7(degrees). Therefore, A,M,D are colinear. Because the angle of CBD is twice the angle of AMC, and because BC=BD, B is the circumcenter of triangle MCD. Therefore CB=BM. Therefore the angle of CMB is 73(degrees)

I think it will be twice of angle CMD, instead of AMC. Great visualisation though.

Prayas Rautray - 3 years, 8 months ago

I did it that way too!

Justin Lee [STUDENT] - 2 years, 5 months ago

1 pending report

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