Video Transcript
In a triangle π΄π΅πΆ, if the measure of angle π΅ is equal to 90 degrees, which of the
following is true? Option (A) π΄π΅ is greater than π΄πΆ. Option (B) π΄π΅ is equal to π΄πΆ. Option (C) π΄πΆ is greater than π΄π΅. Or is it option (D) π΅πΆ is greater than π΄πΆ?
In this question, we are given that the measure of one of the angles in a triangle is
90 degrees. And we need to use this information to determine the correct statement comparing the
side lengths in the triangle.
To answer this question, letβs start by sketching the information we have been
given. We have a triangle π΄π΅πΆ with a right angle at π΅ as shown. We can then note that since π΄π΅πΆ is a right triangle, the side opposite the right
angle will be the hypotenuse, that is, side π΄πΆ. We know that the hypotenuse is the longest side in a right triangle. We can see that this matches option (C), since this states that the length of π΄πΆ is
greater than the length of π΄π΅.
It is also worth noting that none of the other options can be correct, since side
π΄πΆ has greater length than all of the other sides, since it is the hypotenuse.
It is also possible to answer this question without using the properties of right
triangles. To do this, we will use the side comparison theorem in triangles. First, we know that the sum of the measures of the internal angles in a triangle is
180 degrees. So the measure of angle π΄ plus the measure of angle π΅ plus the measure of angle πΆ
is 180 degrees. Next, we know that the measure of angle π΅ is 90 degrees. So we can substitute this into the equation and subtract 90 degrees from both sides
to obtain that the measure of angle π΄ plus the measure of angle πΆ is equal to 90
degrees.
Now we can note that since we have a triangle, the measures of the angles π΄ and πΆ
must be positive. And for the sum of two positive numbers to be equal to 90, both of these numbers must
be less than 90. So the angle at π΅ is the angle with the largest measure in the triangle.
We can now apply the side comparison theorem in triangles that states that the side
opposite the angle with larger measure in a triangle will be longer. Since we have shown that the measure of angle π΅ is greater than the measure of the
other two angles, the side opposite this angle must be longer than the other two
sides. So π΄πΆ is greater than π΅πΆ and π΄πΆ is greater than π΄π΅.
This is a much longer method to show that the answer is option (C). However, there is one useful property that we can obtain from this method. This exact method will work on any triangle with an angle of measure greater than or
equal to 90 degrees. We can show that this angle will have the largest measure in the triangle, so the
side opposite this angle is the longest.
In either case, we have shown that in a triangle π΄π΅πΆ, where the measure of angle
π΅ is 90 degrees, we must have that π΄πΆ is greater than π΄π΅, which is option
(C).