θ = ? \theta=\ ?

Geometry Level 3

Find the larger θ \theta in degrees.


The answer is 68.

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2 solutions

Note that A B C \triangle ABC is isosceles that is A B = A C AB = AC . We have E B A = 18 0 3 8 3 3 = 10 9 \angle EBA = 180^\circ - 38^\circ - 33^\circ = 109^\circ , E A C = 18 0 2 θ 3 3 \angle EAC = 180^\circ - 2 \theta - 33^\circ and E C A = 18 0 ( 18 0 2 θ 3 3 ) 1 9 = 2 θ + 1 4 \angle ECA = 180^\circ - (180^\circ - 2 \theta - 33^\circ) - 19^\circ = 2\theta + 14^\circ . By sine rule ,

{ E A A B = sin 10 9 sin 3 8 . . . ( 1 ) E A A C = sin ( 2 θ + 1 4 ) sin 1 9 . . . ( 2 ) \begin{cases} \dfrac {EA}{AB} = \dfrac {\sin 109^\circ}{\sin 38^\circ} & ...(1) \\ \dfrac {EA}{AC} = \dfrac {\sin (2\theta + 14^\circ)}{\sin 19^\circ} & ...(2) \end{cases}

Since A B = A C AB=AC ,

sin ( 2 θ + 1 4 ) sin 1 9 = sin 10 9 sin 3 8 Note that sin ( 18 0 ϕ ) = sin ϕ sin ( 2 θ + 1 4 ) = sin 7 1 2 cos 1 9 and sin ( 2 ϕ ) = 2 sin ϕ cos ϕ sin ( 2 θ + 1 4 ) = cos 1 9 2 cos 1 9 also sin ϕ = cos ( 9 0 ϕ ) = 1 2 2 θ + 14 = 3 0 or 15 0 θ = 6 or 68 \begin{aligned} \frac {\sin (2\theta + 14^\circ)}{\sin 19^\circ} & = \frac {\blue{\sin 109^\circ}}{\red{\sin 38^\circ}} & \small \blue{\text{Note that }\sin (180^\circ - \phi)= \sin \phi} \\ \sin (2\theta + 14^\circ )& = \frac {\blue{\sin 71^\circ}}{\red{2\cos 19^\circ}} & \small \red{\text{and }\sin (2\phi) = 2\sin \phi \cos \phi} \\ \sin (2\theta + 14^\circ )& = \frac {\blue{\cos 19^\circ}}{2\cos 19^\circ} & \small \red{\text{also }\sin \phi = \cos (90^\circ - \phi)} \\ & = \frac 12 \\ \implies 2\theta + 14 & = 30^\circ \text{ or } 150^\circ \\ \theta & = 6^\circ \text{ or } \boxed{68}^\circ \end{aligned}

In the second-to-last step, how did you choose 15 0 150^\circ instead of 3 0 30^\circ to use as arcsin ( 1 2 ) \arcsin(\frac{1}{2}) ?

Elijah L - 1 year, 2 months ago

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You are right I should choose both.

Chew-Seong Cheong - 1 year, 2 months ago

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Is there a reason why 6 6^\circ is the answer and not 6 8 68^\circ ?

Elijah L - 1 year, 2 months ago

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@Elijah L Both are the right answer.

Chew-Seong Cheong - 1 year, 2 months ago

Let A B C = A C B = θ \angle {ABC}=\angle {ACB}=\theta . Then B C = 2 A B cos θ |\overline {BC}|=2|\overline {AB}|\cos \theta . Let A E B = 38 ° , B A E = 33 ° , A E C = 19 ° \angle {AEB}=38\degree, \angle {BAE}=33\degree, \angle {AEC}=19\degree . Then A B A E = sin 38 ° sin 109 ° = 2 sin 19 ° , A C A E = sin 19 ° sin ( 14 ° + 2 θ ) \dfrac{|\overline {AB}|}{|\overline {AE}|}=\dfrac{\sin 38\degree}{\sin 109\degree}=2\sin 19\degree, \dfrac{|\overline {AC}|}{|\overline {AE}|}=\dfrac{\sin 19\degree}{\sin (14\degree+2\theta)} . A B = A C sin ( 14 ° + 2 θ ) = 1 2 |\overline {AB}|=|\overline {AC}|\implies \sin (14\degree+2\theta)=\dfrac{1}{2} . So θ \theta can be 8 ° 8\degree or 68 ° 68\degree .

@Alak Bhattacharya , I have provided a diagram for you.

Chew-Seong Cheong - 1 year, 2 months ago

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