### Video Transcript

In triangle πππ, the measure of angle π is 90 degrees. ππ equals 15. And ππ equals 25. Triangle πΏππ is similar to triangle πππ, where vertices πΏ, π, and π correspond to vertices π, π, and π, respectively. And each side of triangle πΏππ is one-sixth the length of the corresponding side of triangle πππ. What is the value of cos πΏ?

This question has quite a lot of information. So I think itβs helpful if we try and sketch what we know. We know that angle π is 90 degrees. And that tells us weβre going to be working with a right triangle. Hereβs our right triangle. Angle π measures 90 degrees. And so weβll label this triangle πππ. ππ equals 15. And ππ equals 25. We know that triangle πΏππ is similar to triangle πππ. This means that triangle πΏππ is also a right triangle. We know that πΏ corresponds to π. π corresponds to π. And π corresponds to π.

We are looking for cos of πΏ. We know that cosine relationship equals the adjacent side over the hypotenuse. Something else we can note about similar triangles is that corresponding angles are congruent, which means that angle π is equal to angle πΏ. It also means the cos of π equals the cos of πΏ. Since we know two of the three sides of triangle π, π, and π, we can find the third side in triangle π, π, and π to find the cos of π, which will be equal to the cos of πΏ. Why would we do this? We know that, in triangle πΏππ, each side length is one-sixth the side length of triangle πππ. And that means the length of πΏπ is twenty-five sixths, which we could reduce to four and one-sixth. Either way, dealing with this fraction to find three sides is not as nice as dealing with a whole number.

Now, we know that π squared plus π squared equals π squared. We could set up this equation. 15 squared plus ππ squared equals 25 squared. However, this is a special triangle. Notice that we have three times five. That equals 15. And we have five times five. That equals 25. We recognize that this triangle fits the three-four-five ratio. Three times five is 15. Five times five is 25. If we multiply four by our third side, we find that ππ equals 20. Since we know the cos of π equals the cos of πΏ, we need the adjacent side to π, which measures 20, and the hypotenuse, which is 25. The cos of π equals 20 over 25. And we can reduce this ratio by dividing both sides by five and say the cos of π equals four over five.

Since these two triangles are similar, the cos of πΏ is also four out of five.