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## Trigonometry

### Course: Trigonometry > Unit 1

Lesson 7: The reciprocal trigonometric ratios# Sine & cosine of complementary angles

Learn about the relationship between the sine & cosine of complementary angles, which are angles who together sum up to 90°.

We want to prove that the sine of an angle equals the cosine of its complement.

Let's start with a right triangle. Notice how the acute angles are complementary, sum to 90degrees.

Now here's the cool part. See how the sine of one acute angle

describes the start color #11accd, start text, e, x, a, c, t, space, s, a, m, e, space, r, a, t, i, o, end text, end color #11accd as the cosine of the other acute angle?

Incredible! Both functions, sine, left parenthesis, theta, right parenthesis and cosine, left parenthesis, 90, degrees, minus, theta, right parenthesis, give the exact same side ratio in a right triangle.

And we're done! We've shown that sine, left parenthesis, theta, right parenthesis, equals, cosine, left parenthesis, 90, degrees, minus, theta, right parenthesis.

In other words,

*the sine of an angle equals the cosine of its complement.*Well, technically we've only shown this for angles between 0degrees and 90degrees. To make our proof work for all angles, we'd need to move beyond right triangle trigonometry into the world of unit circle trigonometry, but that's a task for another time.

## Cofunctions

You may have noticed that the words sine and

*co*sine sound similar. That's because they're*co*functions! The way cofunctions work is exactly what you saw above. In general, if f and g are cofunctions, thenand

g, left parenthesis, theta, right parenthesis, equals, f, left parenthesis, 90, degrees, minus, theta, right parenthesis.

Here is a full list of the basic trigonometric cofunctions:

Cofunctions | ||
---|---|---|

Sine and cosine | sine, left parenthesis, theta, right parenthesis, equals, cosine, left parenthesis, 90, degrees, minus, theta, right parenthesis | |

cosine, left parenthesis, theta, right parenthesis, equals, sine, left parenthesis, 90, degrees, minus, theta, right parenthesis | ||

Tangent and cotangent | tangent, left parenthesis, theta, right parenthesis, equals, cotangent, left parenthesis, 90, degrees, minus, theta, right parenthesis | |

cotangent, left parenthesis, theta, right parenthesis, equals, tangent, left parenthesis, 90, degrees, minus, theta, right parenthesis | ||

Secant and cosecant | \sec, left parenthesis, theta, right parenthesis, equals, \csc, left parenthesis, 90, degrees, minus, theta, right parenthesis | |

\csc, left parenthesis, theta, right parenthesis, equals, \sec, left parenthesis, 90, degrees, minus, theta, right parenthesis |

Neat! Whoever named the trig functions must have deeply understood the relationships between them.

## Want to join the conversation?

- how can you study more effectively to do better on tests?(6 votes)
- I'd say just keep practicing problems and remember Soh Cah Toa. I'd also memorize the two triangles.(54 votes)

- So, what kind of questions are related to these confunctions? Im kind of confused because i don't know when to use this(6 votes)
- look sarah when you have to find an hypotenuse knowing an angle(x) and the opposite side you will form an equation like

sin(x)=opp/hyp

therefore sin(x)/opp=1/hyp

therefore opp/sin(x)=hyp

but with cosec funtion you will do it like

cosec(x)=hyp/opp

therefore opp*cosec(x)=hyp

I just wanna conclude saying that cofunctions are helpful when you are giving a math exam without a calculator. Oherwise it is just a concept you should keep in back of your mind.

Keep it simple just remember that

cot or cotan=1/tan(x)

sec or secant = 1/cos(x)

cosec or cosecant=1/sin(x)

I can also give you trick to memorize the trig table for angles 0-90 degrees. Please tell me wether my exaination was clear(23 votes)

- What is Secant?(2 votes)
- Secant (or sec) is the 'flipped version' of the cosine function. If we look at SOH CAH TOA, we see that Cosine is Adjacent over Hypotenuse

but if we use secant, it would be 'flipped' (called the reciprocal), becoming Hypotenuse over Adjacent`cosine =`

Adjacent / Hypotenuse`secant =`

Hypotenuse / Adjacent

Example

so`cos(60 degrees) = 0.5 or 1/2`

`sec(60 degrees) = 2 or 2/1`

(21 votes)

- that was really neat(11 votes)
- Why it can not be
*Sine@=Csc (90-@)***And**

Why it is it**Sine@=Cos(90-@)*bold**(2 votes)`sin`

stands for*sine*.`cos`

stands for*cosine*. cosine is the co-function of sine, which is why it is called that way (there's a 'co' written in front of 'sine'). Co-functions have the relationship`sin@ = cos(90-@)`

However, the trig function`csc`

stands for*cosecant*which is completely different from*cosine*. As you might have noticed,*cosecant*has a 'co' written in front of ''secant'. So we can see here that*cosecant*is the co-function of*secant*. Similarly,*cotangent*is the co-function of*tangent*. Remember that 'co' in front of those names and it'll be much easier to remember them, so that you do not mistaken`csc`

for`cos`

.

A good way to remember is to say the entire name to yourself whenever you're doing trigonometry. For example, say 'cosine' instead of just 'cos' and 'tangent' instead of just 'tan'.(13 votes)

- i still am confused on the topic...(3 votes)
- Welcome to trig!

In all likelihood, if you can keep at it with decent energy levels, you'll understand better as time goes on.(9 votes)

- how do you find other angles with only the sides given?(4 votes)
- With only the sides given, you'd have to solve for an angle using the law of cosines. If the triangle had a right angle, you could use the inverse trig functions. The law of cosines is:

c^2 = a^2 + b^2 - 2*a*c*cos(C)

a, b, and c are sides of a triangle, and C is the angle included between a and b. The law of cosines works by imagining and altitude in the triangle, and basing calculations off of it in such a way that you don't need the altitude measurement to solve the triangle, you just need either all three sides, as in your question, or two sides and the included angle.(5 votes)

- What secant cotangent and cosecant about(3 votes)
- Adding on to ❄RAanchal❄, secant, cosecant, and cotangent are the reciprocal functions of cosine, sine, and tangent. They are not their inverse functions. The inverse functions are arccos, arcsin, and arctan.(5 votes)

- "Neat! Whoever named the trig functions must have deeply understood the relationships between them." Then why is the reciprocal function of Sine, Cosecant and Cosine, Secant. Whoever named the trig functions should've made the reciprocal function of sine, secant and the reciprocal function of Cosine, Cosecant.(5 votes)
- So does sin(θ)=cos(90∘ + theta)?(3 votes)
- sin(𝜃) = cos(90° + 𝜃) is typically not true.

Using the angle addition formula for cosine, we get

sin(𝜃) = cos(90°) cos(𝜃) − sin(90°) sin 𝜃,

which simplifies to

sin(𝜃) = −sin(𝜃) ⇒ sin(𝜃) = 0 ⇒ 𝜃 = 𝑛𝜋, where 𝑛 is an integer.(5 votes)