### Video Transcript

Is π΄π΅πΆπ· a cyclic quadrilateral?

A cyclic quadrilateral is a quadrilateral whose vertices are inscribed on a circle. One way in which we can prove a quadrilateral is cyclic is by checking the angles made with the diagonals. For example, if we could demonstrate that the measure of angle π·π΄πΆ was equal to the measure of angle π·π΅πΆ, then that would show that π΄π΅πΆπ· was cyclic. Another pair of angles we could check would be the measure of angle π΄π·π΅ and the measure of angle π΄πΆπ΅. Showing that these two angle measures are equal would show that the quadrilateral was cyclic. But letβs look at the first pair of angles. This will mean that we need to work out the angle measure of angle π·π΅πΆ.

We have a triangle here π΅πΆπΈ, which will help us, but of course we need two angles in order to find the missing one. Using the fact that the angles on a straight line sum to 180 degrees will allow us to work out this angle measure of πΆπΈπ΅. It will be 180 degrees subtract 83 degrees, which leaves us with 97 degrees. Now, we have the two angles in the triangle, we can calculate this unknown angle measure of π·π΅πΆ.

Because we know that the interior angle measures in a triangle add up to 180 degrees, we have 41 degrees plus 97 degrees plus the measure of angle π·π΅πΆ is equal to 180 degrees. Simplifying the left-hand side, we have 138 degrees plus the measure of angle π·π΅πΆ is 180 degrees. When we subtract 138 degrees from both sides, that leaves us with the measure of angle π·π΅πΆ is 42 degrees. This now demonstrates that we have two equal angle measures. The measure of angle π·π΅πΆ is equal to the measure of angle π·π΄πΆ. And so, π΄π΅πΆπ· is a cyclic quadrilateral.

We could also have investigated the other pair of angles. And we can choose this pair of angles at the diagonals because we were given the angle measure of π΅πΆπ΄. Using the straight line π΄πΆ, we could have worked out that this angle measure at π·πΈπ΄ is also 97 degrees. Using the triangle π΄πΈπ· and the fact that the angles add up to 180 degrees would have allowed us to show that angle π΄π·π΅ is 41 degrees. This would also show that an angle made with a diagonal and side is equal to the angle made with the other diagonal and opposite side. Note that we donβt need to show that both pairs of angles are congruent. Just one of these is sufficient to prove that π΄π΅πΆπ· is cyclic. And so, we can give the answer yes, π΄π΅πΆπ· is a cyclic quadrilateral.