An interesting angle

Geometry Level 3

Find the measure of angle x x in degrees.

20 40 30 60

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2 solutions

Michael Mendrin
Dec 18, 2016

Draw in (red) lines A F , D F , C G AF, DF, CG at angles and between points as shown in graphic. Triangle D G F DGF is equilateral, and all other integer angles as shown are easily found through angle chasing.

Because triangle A F C AFC is isosceles, A F = C F AF=CF . From this, we know that triangles A F E AFE and C F G CFG are congruent.

Therefore G F = E F GF=EF , and since D F = G F DF=GF , we know D F = E F DF=EF , and conclude that triangle D F E DFE is isosceles.

Therefore, D E F = 50 \angle DEF=50 , and from this we have A E D = 20 \angle AED=20

C E B = 180 70 20 60 = 30 \angle CEB = 180-70-20-60=30

B D C = 180 60 10 70 = 40 \angle BDC = 180 - 60-10-70=40

Let C B = 1 CB=1 . Sine law on B C D \triangle BCD .

C D sin 60 = 1 sin 40 \dfrac{CD}{\sin 60}=\dfrac{1}{\sin 40} \implies C D 1.347 CD\approx 1.347

Sine law on B C E \triangle BCE .

C E sin 80 = 1 sin 30 \dfrac{CE}{\sin 80}=\dfrac{1}{\sin 30} \implies C E 1.97 CE \approx 1.97

Cosine law on C D E \triangle CDE .

D E 2 = 1.9 7 2 + 1.34 7 2 2 ( 1.97 ) ( 1.347 ) ( cos 10 ) DE^2=1.97^2+1.347^2-2(1.97)(1.347)(\cos 10) \implies D E 0.685 DE \approx 0.685

Sine law on C D E \triangle CDE .

sin x 1.347 = sin 10 0.685 \dfrac{\sin x}{1.347}=\dfrac{\sin 10}{0.685} \implies x = 20 x=20

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