In triangle $ABC$, $\angle C = 48^\circ$. $D$ is any point on $BC$, such that $\angle CAD = 18^\circ$ and $AC = BD$. Find $\angle ABD.$
In triangle $ABC$, $\angle C = 48^\circ$. $D$ is any point on $BC$, such that $\angle CAD = 18^\circ$ and $AC = BD$. Find $\angle ABD.$
I tried to make some constructions: draw a line through $D$ parallel to $AC$; draw a line through $C$ which makes $66$ degrees with $AC$. None of them have been useful. It feels like I am trying to force the formation of congruent triangles, and that doesn't work. Please help.
5 Answers
$\begingroup$Draw in black the triangle $ABC$ with angle $48^\circ$ at $C$, and $AD$ with angle $18^\circ$ at $A$. This ensures the blue angles $114^\circ$ and $66^\circ$. Choose $D'\in BC$ such that $|D'C|=|BD|=|AC|$. This gives the red angles $66^\circ$ and $66^\circ-18^\circ=48^\circ$. This implies that $|AD'|=|AD|$. The triangles $AD'C$ and $ADB$ are therefore SAS-congruent with the same angles at $D'$ and $D$. This shows that the green angle $\angle ABD=48^\circ$.
Let $E$ be a point such $ED=AD$ and $EB=CD$ and that $A, E$ lie on the same side of line $BC$. Then $\triangle DEB \equiv \triangle ADC$ so $\angle BDE = \angle CAD$ and therefore$$\angle EDA = \angle BDA - \angle BDE = \angle DCA + \angle CAD - \angle BDE = \angle DCA = 48^\circ.$$Since $ED=AD$, we have $$\angle AED = 90^\circ - \frac 12 \angle EDA = 90^\circ - \frac 12 \cdot 48^\circ = 66^\circ.$$But also $\angle DEB = \angle ADC = 180^\circ - 48^\circ - 18^\circ = 114^\circ$. Hence $\angle AED + \angle DEB = 180^\circ$ and therefore $E$ lies on $AB$. It follows that$$\angle ABD = \angle EBD = \angle DCA = 48^\circ.$$
Special thanks to Calum Gilhooley for providing a picture.
$\endgroup$ 2 $\begingroup$One way to solve it would be using sine rule. In ACD, we have
$$\frac{\sin 114}{AC} = \frac{\sin 48}{AD}$$
In triangle ABD, we have
$$\frac{\sin(114-B)}{BD} = \frac{\sin B}{AD}$$
If you solve these equations, using AC=BD, we have$$\tan B = \frac{\sin 48}{1+\cot 114\cdot\sin48}$$
$$\implies \tan B = 1.1107$$
Hence $B = 48^0$
$\endgroup$ 1 $\begingroup$Ok, here's my method, it's VERY confusing and messy, so there's probably a better method. Using the sine rule: $$\frac{AC}{\sin114}=\frac{AD}{\sin 48}$$so $$AC=BD=\frac{AD\sin114}{\sin48}$$Cosine rule: $$AB^2=AD^2+\frac{AD^2\sin^2 114}{\sin^2 48}-2\frac{AD^2\sin 114\cos 66}{\sin 48}=AD^2 (1+\frac{\sin^2 114}{\sin^2 48}-2\frac{\sin 114\cos 66}{\sin 48})$$So $$AB=AD \sqrt{1+\frac{\sin^2 114}{\sin^2 48}-2\frac{\sin 114\cos 66}{\sin 48}}$$Sine rule (again): $$\frac{AB}{\sin66}=\frac{AD}{\sin ABD}$$so $$\frac{AD \sqrt{1+\frac{\sin^2 114}{\sin^2 48}-2\frac{\sin 114\cos 66}{\sin 48}}}{\sin66}=\frac{AD}{\sin ABD}$$Cancelling $AD$ from both ides and rearranging to find $ABC$ should get you your answer. I hope that helped! (It'd probably help to draw a sketch.)
$\endgroup$ 3 $\begingroup$Let $\measuredangle ABC=x$. Then $\measuredangle BAD=114^\circ-x$. By sine theorem we have:$$ BD\stackrel{\triangle BAD}=AD\frac{\sin(114^\circ-x)}{\sin x} \stackrel{\triangle CAD}=AC\frac{\sin 48^\circ}{\sin 114^\circ}\frac{\sin(114^\circ-x)}{\sin x}\\ \implies \frac{\sin 48^\circ}{\sin 114^\circ}\frac{\sin(114^\circ-x)}{\sin x}=1 \implies \frac{\sin x}{\sin(114^\circ-x)}=\frac{\sin 48^\circ}{\sin 66^\circ} \implies x=48^\circ. $$
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