# Math Help - Angles in cyclic quadrilateral in semi-circle

1. ## Angles in cyclic quadrilateral in semi-circle

Given a cyclic quadrilateral ABCD in a semi-circle, (let AD be the diameter),let AB=BC=1. Draw the line DB.
Prove that angle ADB= angle BDC

This was taken as obvious in a solution I read, but it is not at all obvious to me. Help in understanding this result would be appreciated.

2. ## Re: Angles in cyclic quadrilateral in semi-circle

Hello, I-Think!

$\text{Given a cyclic quadrilateral }ABCD\text{ in a semicircle with diamter AD.}$
$\text{Let }AB = BC = 1.\;\text{ Draw the line segment }DB.$
$\text{Prove that: }\:\angle ADB \,=\, \angle BDC$
Didn't they explain why it is "obvious"?

Angles $ADB$ and $BDC$ are inscribed angles.
. . They are measured by one-half their intercepted arcs.

We are told that chords $AB$ and $BC$ are equal.
. . Hence, their respective arcs are equal: . $\overline{AB} \:=\:\overline{BC}$

Therefore: . $\angle ADB \,=\,\angle BDC$