Oh yes!

Geometry Level 4

The B \angle B of an isosceles triangle A B C ABC is 11 0 110^\circ . Inside the triangle, a point M M is taken such that M A C = 3 0 \angle MAC=30^\circ and M C A = 2 5 \angle MCA=25^\circ . Find B M C \angle BMC in degrees.


The answer is 85.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Let B M C = θ \angle BMC = \theta , B C = a BC = a , A C = b AC = b and M C = d MC = d . By sine rule, we have:

{ a b = sin A sin B = sin 3 5 sin 11 0 b d = sin A M C sin M A C = sin 12 5 sin 3 0 a d = sin M B C sin B M C = sin θ sin ( 17 0 θ ) \begin{cases} \dfrac{a}{b} = \dfrac{\sin \angle A}{\sin \angle B} & = \dfrac{\sin 35^\circ}{\sin 110^\circ} \\ \dfrac{b}{d} = \dfrac{\sin \angle AMC}{\sin \angle MAC} & = \dfrac{\sin 125^\circ}{\sin 30^\circ} \\ \dfrac{a}{d} = \dfrac{\sin \angle MBC}{\sin \angle BMC} & = \dfrac{\sin \theta}{\sin (170^\circ-\theta)} \end{cases}

a d = a b × b d sin θ sin ( 17 0 θ ) = sin 3 5 sin 12 5 sin 11 0 sin 3 0 = cos ( 9 0 3 5 ) sin ( 18 0 12 5 ) 2 sin 5 5 cos 5 5 × 1 2 = cos 5 5 sin 5 5 sin 5 5 cos 5 5 = 1 sin θ = sin ( 17 0 θ ) θ = 17 0 θ 2 θ = 17 0 θ = B M C = 85 \begin{aligned} \frac{a}{d} & = \frac{a}{b}\times \frac{b}{d} \\ \Rightarrow \frac{\sin \theta}{\sin (170^\circ-\theta)} & = \frac{\sin 35^\circ \sin 125^\circ}{\sin 110^\circ \sin 30^\circ} \\ & = \frac{\cos (90^\circ - 35^\circ) \sin (180^\circ - 125^\circ)}{2\sin 55^\circ \cos 55^\circ \times \frac{1}{2}} \\ & = \frac{\cos 55^\circ \sin 55^\circ}{\sin 55^\circ \cos 55^\circ} \\ & = 1 \\ \Rightarrow \sin \theta & = \sin (170^\circ - \theta) \\ \theta & = 170^\circ - \theta \\ 2 \theta & = 170^\circ \\ \Rightarrow \theta & = \angle BMC = \boxed{85}^\circ \end{aligned}

Yellow Tomato
Nov 5, 2015

Just a tip, you can do degrees with ^{ \circle}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...