### Video Transcript

In the figure, the line segment π΄π΅ lies in the plane π₯ and the line segment π΄πΆ is perpendicular to π₯. Given that π΄π΅ equals six and π΄πΆ equals eight, find the length of the line segment π΅πΆ.

Letβs begin by adding what we know to our diagram. Weβre told that π΄π΅ is equal to six. So the line segment π΄π΅ must be six units in length. Similarly, π΄πΆ is equal to eight. So the line segment π΄πΆ is eight units. Weβre looking to find the length of the line segment π΅πΆ, as shown. So we need to use the final piece of information in our question. Weβre told that the line segment π΄π΅ lies in the plane π₯. And the line segment π΄πΆ is perpendicular to that same plane. This means the line segment π΄π΅ must be perpendicular to the line segment π΄πΆ. That is, π΄π΅ and π΄πΆ form a right angle.

Iβve redrawn the right-angle triangle so we can see whatβs happening in a little bit more detail. Labelling the unknown side π, we see that we know the length of the short two sides in the right-angle triangle. And π is the hypotenuse. We can therefore use the Pythagorean theorem to work out the length of the missing side. This says that the sum of the squares of the short two sides must be equal to the square of the longest side. Itβs usually written as π squared plus π squared equals π squared. Of course, in our triangle, the longest side is labelled π. So we could say that π squared plus π squared equals π squared. Substituting the values from our triangle in, and we find that eight squared plus six squared equals π squared.

Now, you might recognize a Pythagorean triple hit. But weβre going to solve the equation just in case. Eight squared is 64. And six squared is 36. Their sum is 100. So we find π squared is equal to 100. We solve this equation by finding the square root of 100. Remember, we donβt need to find both a positive and negative square root since weβre dealing with a length. It absolutely must be positive. And when we find the square root of 100, we get 10. The length of π΅πΆ is 10 units. Now, in fact, six, eight, and 10 form a Pythagorean triple. We know that six squared plus eight squared equals 10 squared. So we couldβve deduced quite quickly that π was equal to 10. It is, of course, absolutely fine to solve the equation.