Find x x^\circ

Geometry Level 3

Find x x in degrees.


The answer is 25.

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1 solution

A D C = 55 ° , B C A = B A C = 35 ° , C A D = A D B = 30 ° \angle {ADC}=55\degree, \angle {BCA}=\angle {BAC}=35\degree, \angle {CAD}=\angle {ADB}=30\degree . Therefore B D C = A D C A D B = 55 ° 30 ° = 25 ° \angle {BDC}=\angle {ADC}-\angle {ADB}=55\degree-30\degree=\boxed {25\degree} ( A B C , B C D \triangle {ABC},\triangle {BCD} are isosceles triangles).

I don't understand how angle CAD and ADB are equal. Can you please explain this part.

Sabhrant Sachan - 1 year, 2 months ago

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You can easily get A C = 2 A D cos 35 ° |\overline {AC}|=2|\overline {AD}|\cos 35\degree . Apply sine rule to B A D \triangle {BAD} to obtain A B = A D × sin A D B sin A B D |\overline {AB}|=|\overline {AD}|\times \dfrac{\sin \angle {ADB}}{\sin \angle {ABD}} , with A D B = 60 ° + x , A B D = 55 ° x \angle {ADB}=60\degree+x, \angle {ABD}=55\degree-x . Applying sine rule to A B C \triangle {ABC} you get A B = A C × sin 95 ° sin 55 ° |\overline {AB}|=|\overline {AC}|\times \dfrac{\sin 95\degree}{\sin 55\degree} . Comparing the two expressions for A B |\overline {AB}| you can get 55 ° x = 30 ° 55\degree-x=30\degree .

A Former Brilliant Member - 1 year, 2 months ago

How do you know that triangle A B C ABC and triangle B C D BCD are isosceles?

Joshua Lowrance - 1 year, 2 months ago

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Because D B C = 25 ° B D C = 50 ° 25 ° = 25 ° \angle {DBC}=25\degree\implies \angle {BDC}=50\degree-25\degree=25\degree .

A Former Brilliant Member - 1 year, 2 months ago

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