# Question Video: Finding the Length of Chord in a Circle by Applying the Pythagorean Theorem Mathematics

Given that ππ = 42 cm and ππ΄ = 58 cm, find the length of line segment π΄π΅.

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### Video Transcript

Given that ππ equals 42 centimeters and ππ΄ equals 58 centimeters, find the length of line segment π΄π΅.

Letβs start by filling in the measurement information that we were given. So ππ is 42 centimeters and ππ΄ is 58 centimeters. We are asked to work out the length of the line segment π΄π΅. We should notice that the line segment π΄π΅ is a chord. Thatβs because itβs a line segment which joins two distinct points on the circumference. There is also this line ππ, which is a line from the center π to the chord. We can also see that there is a right angle of 90 degrees, meaning that this line from the center meets the chord at 90 degrees.

We should recall the property that if we have a circle with center π΄ containing a chord, the line segment π΅πΆ, then the straight line that passes through π΄ and is perpendicular to line segment π΅πΆ also bisects line segment π΅πΆ. The application of this property here means that the line segment π΄π is congruent to the line segment ππ΅. This is because the chord π΄π΅ has been bisected. That means that if we knew the length of either the line segment π΄π or the line segment ππ΅, then we could double it to find the length of the line segment π΄π΅.

So letβs consider this triangle π΄ππ. And we know that this is going to be a right triangle because the angle measure of π΄ππ will also be 90 degrees. This comes, of course, from the fact that we have a straight line π΄π΅ and the angles on a straight line sum to 180 degrees.

So now we have a right triangle, and we know two of the lengths in this right triangle, and we want to find the length of the third side. So letβs define the length of the line segment π΄π to be π¦ centimeters. We can find the value of π¦ by applying the Pythagorean theorem. This theorem tells us that in any right triangle, the square of the hypotenuse is equal to the sum of the squares on the other two sides.

And so we have the hypotenuse squared. The hypotenuse is always opposite the right angle. So 58 squared is equal to 42 squared plus π¦ squared. 58 squared is equal to 3364, and 42 squared is equal to 1764. Subtracting 1764 from both sides leaves us with 1600 is equal to π¦ squared.

Next, we must take the square root of both sides of this equation, remembering that since π¦ is a length, then this will be the positive value of the square root. And so we have that 40 is equal to π¦. And so we know that the length of the line segment π΄π is 40 centimeters.

We must be careful here. We havenβt finished the question because we were asked for the length of the line segment π΄π΅. This is where our property becomes very important. We know that the line segment π΄π is equal to the line segment π΅π. So we can find the length of the line segment π΄π΅ by doubling the length of the line segment π΄π. Thatβs two times 40, which is 80. And so we can give the answer that the length of line segment π΄π΅ is 80 centimeters.