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Finding reciprocal trig ratios

Sal finds all six trigonometric ratios (sine, cosine, tangent, secant, cosecant, and cotangent) of an angle in a given right triangle. Created by Sal Khan.

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  • piceratops ultimate style avatar for user shabarish.ch
    What is the full form of csc, sec and cot?
    (77 votes)
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  • mr pink red style avatar for user Ishaan Pota
    Okay,so now we know how to calculate sin cosin and tangent.But where do we use them!!and how are these formulae derived?
    (17 votes)
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  • leaf green style avatar for user Qrious
    When would one want to use csc, sec and cot instead of sin, cos and tan?
    (19 votes)
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    • piceratops ultimate style avatar for user RahulAroraDFS
      It's just for one's knowledge: also, when one has the angle and the opposite side and is trying to calculate the adjacent, it is easier to simplify the cotangent function than the tangent - this is also true for the other trig ratios trigx=a/b when you need to find b.

      cot(theta)=adjacent/opposite
      opposite(cot(theta))=adjacent

      than it is to simplify

      tan(theta)=opposite/adjacent
      adjacent(tan(theta))=opposite
      adjacent=opposite/(tan(theta))

      I hope that makes sense!
      (15 votes)
  • leafers seedling style avatar for user Jamie
    is there any easy mnemonic device to remember which reciprocal functions and normal trig functions go together?
    (13 votes)
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    • starky sapling style avatar for user a.david.rose
      Personally I just imagine that the reciprocal functions aren't allowed to start with the same letter as big three do:

      sine and secant both start with s, so the inverse must be cosecant.
      cosine and cosecant both start with c, so the inverse must be secant.
      tangent and cotangent already start with different letters, so it works.

      That's how my mind works, anyway. Maybe yours will agree!
      (21 votes)
  • blobby green style avatar for user Tom
    how do you do the CscA, secA amd CotA on a calculator?
    (9 votes)
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  • ohnoes default style avatar for user Rohen Giralt
    What does reciprocal mean?
    (9 votes)
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    • male robot hal style avatar for user Kunal Chugh
      1. In reciprocal you have to take an integer (like 6) and then convert it into a fraction. In this case it would be 6/1.
      2. Then switch the numerator and denominator. So your answer would be 1/6.

      If the number is already fraction then just do step 2.

      Hope this helps!

      By the way: opposite reciprocal is the same thing, just change the positive sign to a negative or a negative sign to a positive.
      (13 votes)
  • blobby green style avatar for user Tarun G Maddila
    how can we say that sin square plus cos square equals to one
    (8 votes)
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    • female robot grace style avatar for user cjddowd
      The cosine is adjacent over hypotenuse.The sine is opposite over adjacent. So:
      Cos^2(x)+sin^2(x)
      (adj^2/hyp^2) + (opp^2/hyp^2)
      (adj^2/opp^2)/hyp^2
      hyp^2/hyp^2 (because of Pythagorean theorem)
      1
      A similar process can be done for sec^2(x)-tan^2(x)=1 and csc^2(x)-cot^2(x)=1
      (6 votes)
  • purple pi teal style avatar for user Alderkit
    Isn't this the same thing as the arcsine, arcosine, and arctangent?
    (3 votes)
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    • blobby green style avatar for user David Calkins
      Definitely NOT. Reciprocal trig ratios are NOT the same thing as the arcsin, arccosine, and arctangent. I was screwed up by this when I was first learning trig, and it's why I really hate the notation they use to describe these concepts.

      So in the video, he's talking about reciprocal ratios. Remember that when you figure out a value's reciprocal, you just "flip" it as a fraction; the numerator becomes the denominator and vice versa. And all trig functions are just ratios (fractions), so their reciprocal ratios are just the initial function "flipped." Sine is "opposite over hypotenuse (o/h)" and its reciprocal ratio is cosecant which is "hypotenuse over opposite (h/o)."

      What's mixing you up is that you probably know from algebra that anything to the power of -1 has the effect of generating a reciprocal. (3/4)^-1 = 4/3. So it makes sense that what looks like sin^-1 (x) would = 1/sin(x), which is cosecant, right?

      Wrong, unfortunately. What looks like sin^-1(x) is actually ARCSIN which is NOT = cosecant. Don't ask me why. It's the darn notation that's screwy - not you.

      Arcsin works a lot like logarithms work, if you're familiar with those. If you ask arcsin(0.5) = ?, what you're really asking is "the sine of what angle equals .5?" In this case, the answer is pi/6 radians. (or 30 degrees). (and also 5pi/6).

      sin(pi/6) = 0.5
      arcsin(0.5) = pi/6 (radians)

      Unfortunately, for some bizarre reason, math has chosen to represent arcsin (and the other arcfunctions) with the -1 power sign, which just confuses everything.

      For the most part, you will NEVER have to deal with negative powers of trig functions. Squares, definitely - but never negative values. So if you ever see that -1 after sin, cos, or tan, just remember it represents ARCsin, ARCcos, and ARCtan and NOT the reciprocal trig functions.
      (14 votes)
  • duskpin ultimate style avatar for user Kyle
    Are the csc,sec,and cot really that helpful anyway?
    (6 votes)
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  • duskpin ultimate style avatar for user Vishal Mishra
    hey then sal in previous video took something as tan to power of -1 but could not he take it as cot
    (4 votes)
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Video transcript

Determine the six trigonometric ratios for angle A in the right triangle below. So this right over here is angle A, it's at vertex A. And to help me remember the definitions of the trig ratios-- and these are human constructed definitions that have ended up being very, very useful for analyzing a whole series of things in the world. To help me remember them, I use the words soh cah toa. Let me write that down. Soh cah toa. Sometimes you can think of it as one word, but it's really the three parts that define at least three of the trig functions for you. And then we can get the other three by looking at the first three. So soh tells us that sine of an angle-- in this case it's sine of A-- so sine of A is equal to the opposite, that's the O, over the hypotenuse. Well in this context, what is the opposite side to angle A? Well, we go across the triangle, it opens up onto side BC. It has length 12. So that is the opposite side. So, this is going to be equal to 12. And what's the hypotenuse? Well, the hypotenuse is the longest side of the triangle. It's opposite the 90 degree angle. And so we go opposite the 90 degree angle, longest side is side AB. It has length 13. So this right over here is the hypotenuse. So, the sine of A is 12/13. Now let's go to cah. Cah defines cosine for us. It tells us that cosine of an angle-- in this case, cosine of A-- is equal to the adjacent side to the angle over the hypotenuse. So, what's the adjacent side to angle A? Well, if we look at angle A, there are two sides that are next to it. One of them is the hypotenuse. The other one has length 5. The adjacent one is side CA. So it's 5. And what is the hypotenuse? Well, we've already figure that out. The hypotenuse is right over here, it's opposite the 90 degree angle. It's the longest side of the right triangle. It has length 13. So the cosine of A is 5/13. And let me label this. This right over here is the adjacent side. And this is all specific to angle A. The hypotenuse would be the same regardless of what angle you pick, but the opposite and the adjacent is dependent on the angle that we choose in the right triangle. Now let's go to toa. Toa defines tangent for us. It tells us that the tangent of an angle is equal to the opposite side over the adjacent side. So given this definition, what is the tangent of A? Well, the opposite side, we already figured out, has length 12. And the adjacent side, we already figure out, has length 5. So the tangent of A, which is opposite over adjacent, is 12/5. Now, we'll go the to the other three trig ratios, which you could think of as the reciprocals of these right over here. But I'll define it. So first you have cosecant. And cosecant, it's always a little bit unintuitive why cosecant is the reciprocal of sine of A, even though it starts with a co like cosine. But, cosecant is the reciprocal of the sine of A. So sine of A is opposite over hypotenuse. Cosecant of A is hypotenuse over opposite. And so what's the hypotenuse over the opposite? Well, the hypotenuse is 13 and the opposite side is 12. And notice that 13/12 is the reciprocal of 12/13. Now, secant of A is the reciprocal. So instead of being adjacent over hypotenuse, which we got from the cah part of soh cah toa, it's hypotenuse over adjacent. So what is the secant of A? Well, the hypotenuse, we've figured out multiple times already, is 13. And what is the adjacent side? It's 5. So it's 13/5, which is, once again, the reciprocal of the cosine of A, 5/13. Finally, let's get the cotangent. And the cotangent is the reciprocal of tangent of A. Instead of being opposite over adjacent, it is adjacent over opposite. So what is the cotangent of A? Well, we've figured out the adjacent side multiple times for angle A. It's length 5. And the opposite side to angle A is 12. So it's 5/12, which is, once again, the reciprocal of the tangent of A, which is 12/5.