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Integrated math 3

Course: Integrated math 3>Unit 8

Lesson 2: Law of cosines

Solving for a side with the law of cosines

Learn how to use the law of cosines to find the missing side length of a triangle when given two side lengths and the contained angle measure. Created by Sal Khan.

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• Maybe I'm just not quite getting this, but why not just use the Pythagorean Theorem? It comes out to 15, right?
• You can ONLY use the Pythagorean Theorem when dealing with a right triangle. The law of cosines allows us to find angle (or side length) measurements for triangles other than right triangles. The third side in the example given would ONLY = 15 if the angle between the two sides was 90 degrees. In the example in the video, the angle between the two sides is NOT 90 degrees; it's 87. As such, that opposite side length isn't 15; it's 14.6.
• What's the difference between a theorem and a law?
• Good question!
In science at least, here is the difference between a theory/theorem and a law:
A theory is an explanation for a natural occurrence. It tells the "why" about something, but it has not necessarily been proven.
A law, on the other hand, states a fact- something that always happens. It tells the "what" without explaining why, and it should always be true.
• Is there a Law of Tangent?
• Yes, you can find it on Wikipedia. But it's equivalent to the Law of Sines, so it's not really useful.
• in the equation,a^2=b^2+c^2-2bc cos(theta),does a have to be the longest side
or can it be any side
• "a" in the law of cosines is the side opposite of the angle theta, so it can be of any length.
• In what situation do you use the law of cosines?
• You could use it if you know SSS and want to find an angle, or if you know SAS and want to find the remaining side.
• At couldn't you just use the Pythagorean Theorem?
• No, because it's not a right triangle (or, at the very least, we can't prove it to be a right triangle).
• why is it whenever sal khan explains it's hard to understand?
• Some people have an easier time with spoken explanations, or written, or demonstrated
• WHy are they assigning this in preschool
• Lol, I am assigned as the teacher for my brothers and sometimes for fun I would assign them tasks that they couldn't do. XD That was a few years back.