Find the missing angle

Geometry Level 3

In the above diagram, B A = A E = E D = D C BA=AE=ED=DC , find x x (in degrees). Round off your answer to the nearest integer.


The answer is 112.

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1 solution

Let B A = A E = E D = D C = 1 BA=AE=ED=DC=1 . Since E D C \triangle EDC is isosceles, D E C = E C D = 40 \angle DEC=ECD=40 . Also, E A B \triangle EAB is isosceles so A B E = B E A = 25 \angle ABE=BEA=25 . Now B E C = 120 25 40 = 55 \angle BEC=120-25-40=55 .

By law of cosines on E D C \triangle EDC , we have

w 2 = 1 2 + 1 2 2 ( 1 ) ( 1 ) ( cos 100 ) w^2=1^2+1^2-2(1)(1)(\cos~100) \implies w = 1.5321 w=1.5321

By law of cosines in E A B \triangle EAB , we have

y 2 = 1 2 + 1 2 2 ( 1 ) ( 1 ) ( cos 130 ) y^2=1^2+1^2-2(1)(1)(\cos~130) \implies y = 1.8126 y=1.8126

By law of cosines in B E C \triangle BEC , we have

z 2 = 1.812 6 2 + 1.532 1 2 2 ( 1.8126 ) ( 1.5321 ) ( cos 55 ) z^2=1.8126^2+1.5321^2-2(1.8126)(1.5321)(\cos~55) \implies z = 1.56432 z=1.56432

By law of sines in B E C \triangle BEC , we have

sin θ 1.8426 = sin 55 1.5643 \dfrac{\sin~\theta}{1.8426}=\dfrac{\sin~55}{1.5643}

θ = 71.65 \theta=71.65

Finally, x = E C D + θ = 40 + 71.65 = 111.65 x=\angle ECD+\theta=40+71.65=111.65 \approx 112 \boxed{112}

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