Find x x

Geometry Level 4

Find x x in degrees.

Clarifications:

  • The figure above is not drawn to scale.
  • The angles shown are in degrees.


The answer is 30.

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1 solution

By trigonometry.

A E B = 180 60 80 = 40 \angle AEB=180-60-80=40

A D B = 180 80 50 = 50 \angle ADB=180-80-50=50

A D B \triangle ADB is isosceles.

Let A B = A D = 1 AB=AD=1 .

By sine law on A D B \triangle ADB , we have

D B sin 80 = 1 sin 50 \dfrac{DB}{\sin 80}=\dfrac{1}{\sin 50}

D B = sin 80 sin 50 DB=\dfrac{\sin 80}{\sin 50}

By sine law on A E B \triangle AEB , we have

E B sin 60 = 1 sin 40 \dfrac{EB}{\sin 60}=\dfrac{1}{\sin 40}

E B = sin 60 sin 40 EB=\dfrac{\sin 60}{\sin 40}

By cosine law on D E B \triangle DEB , we have

D E 2 = ( sin 60 sin 40 ) 2 + ( sin 80 sin 50 ) 2 2 ( sin 60 sin 40 ) ( sin 80 sin 50 ) ( cos 30 ) DE^2=\left(\dfrac{\sin 60}{\sin 40}\right)^2+\left(\dfrac{\sin 80}{\sin 50}\right)^2-2\left(\dfrac{\sin 60}{\sin 40}\right)\left(\dfrac{\sin 80}{\sin 50}\right)(\cos 30)

D E 0.68404 DE \approx 0.68404

By sine law on A D E \triangle ADE , we have

sin x 1 = sin 20 0.68404 \dfrac{\sin x}{1}=\dfrac{\sin 20}{0.68404}

x = 3 0 \boxed{x=30^\circ}

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