Answer:
[tex]\approx 15.9[/tex]
Step-by-step explanation:
The length of an arc with measure [tex]\theta[/tex] and radius [tex]r[/tex] is given by [tex]\ell_{arc}=2r\pi\cdot \frac{\theta}{360}[/tex]. From the figure, we know that the radius of arc ADC is 4, but we don't know the measure of the arc. Since there are 360 degrees in a circle, the measure of arc ADC is equal to the measure of the arc formed by [tex]\angle AOC[/tex] subtracted from 360. The measure of the arc formed by [tex]\angle AOC[/tex] consists of two congruent angles, [tex]\angle AOB[/tex] and [tex]\angle COB[/tex]. To find them, we can use basic trigonometry for a right triangle, since by definition, tangents intersect a circle at a right angle.
In any right triangle, the cosine of an angle is equal to its adjacent side divided by the hypotenuse, or longest side, of the triangle.
We have:
[tex]\cos \angle AOB=\cos \angle COB=\frac{4}{10},\\\angle AOB=\arccos(\frac{4}{10})=66.42182152^{\circ}[/tex]
Therefore, [tex]\angle AOC=2\cdot 66.42182152=132.84364304^{\circ}[/tex]
The measure of the central angle of [tex]\widehat{ADC}[/tex] must then be [tex]360-132.84364304=227.15635696^{\circ}[/tex]
Thus, the length of [tex]\widehat{ADC}[/tex] is equal to:
[tex]\ell_{\widehat{ADC}}=2\cdot 4\cdot \pi \cdot \frac{227.15635696}{360},\\\ell_{\widehat{ADC}}=15.8585053832\approx \boxed{15.9}[/tex] (three significant figures as requested by question).
