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## Precalculus

### Course: Precalculus>Unit 2

Lesson 10: Using trigonometric identities

# Using the tangent angle addition identity

Find the tangent of 13pi/12 without a calculator using the tangent angle addition identity. Created by Sal Khan.

## Want to join the conversation?

• In , Sal said we've proven tangent identities in another video. Where can I find that video?

I was trying to search the tangent identities but couldn't find a video with the sum and difference identities of the tangent.
• Why did Sal rationalize the variables last? I mean, I can see why that was a good idea since it was faster than just rationalizing them from the start, but is there a rule on when to rationalize or is it just common sense on when it's a good idea to rationalize now than later?

Edit: I notice he didn't have to rationalize this way.
• It's always easier to work with smaller numbers, so personally I lean to rationalizing denominators afterwards unless I see some obvious benefit of doing it beforehand.
• I have been using ChatGPT for useful discussions. But remember it is not infallible. It makes mistakes that it owns up to when you point it out
(1 vote)
• When Sal says the slope of the tangent on the unit circle is just the radius for 5pi/4 = 1, why is it different for pi/6?
(1 vote)
• Since tangent is opposite divided by adjacent, the values in the 45-45-90 triangle are both the same so it'll just be 1.
(1 vote)
• How would you solve an equation such as (cos(x))/(1+csc(x))*(1-csc(x))/(1-csc(x))?
I end up always getting these wrong, and they always answer (such as with this one) with something like tan(x)-tan(x)sin(x). why and how did they get to that answer??
(1 vote)
• why are the practice problems so much harder than what we learn in the video?
i feel like there are not enough videos here to go ahead to the parctice problem because he does not show how to solve so many of them