Bridge to Nowhere

Geometry Level 4

In a quadrilateral A B C D ABCD , A D B = 4 1 , C A B = 4 2 , A B D = 4 3 \angle ADB=41^\circ, \angle CAB=42^\circ, \angle ABD=43^\circ , and A C B = 4 4 \angle ACB=44^\circ . Find A C D \angle ACD to 2 decimal places.


The answer is 45.55.

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1 solution

Marta Reece
Jan 15, 2017

A large number of angles can be easily obtained and some are shown in the image above. To find x x , we can arbitrarily set D E = 1 \overline{DE}=1 . From there we can go over the bridge to C E \overline{CE} using the law of sines three times for the triangles A E D , A B E , B C E \triangle AED, \triangle ABE, \triangle BCE .

A E sin ( 41 ) = 1 sin ( 54 ) , B E sin ( 42 ) = A E sin ( 43 ) , C E sin ( 51 ) = B E sin ( 44 ) \frac{\overline{AE}}{\sin(41)}=\frac{1}{\sin(54)}, \frac{\overline{BE}}{\sin(42)}=\frac{\overline{AE}}{\sin(43)}, \frac{\overline{CE}}{\sin(51)}=\frac{\overline{BE}}{\sin(44)}

C E = sin ( 51 ) sin ( 44 ) × sin ( 42 ) sin ( 43 ) × sin ( 41 ) sin ( 54 ) = 0.89011 \overline{CE}=\frac{\sin(51)}{\sin(44)}\times\frac{\sin(42)}{\sin(43)}\times\frac{\sin(41)}{\sin(54)}=0.89011

In the C D E \triangle CDE the C E D = 9 5 \angle CED=95^\circ and the sides next to it are 1 and 0.89011. C D \overline{CD} can be obtained from the law of cosines as

C D = 1 + 0.8901 1 2 2 × 0.89011 × cos ( 95 ) = 1.3955 \overline{CD}=\sqrt{1+0.89011^2-2\times0.89011\times \cos(95)}=1.3955

The law of sines applied to the C D E \triangle CDE provides x x as

x = arcsin ( sin ( 95 ) 1.3955 ) = 45.5 5 x= \arcsin(\frac{\sin(95)}{1.3955})=45.55^\circ

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