Trigonometry/Cosecant, Secant, Cotangent
The cosecant (), secant () and cotangent () functions are 'convenience' functions, just the reciprocals of (that is 1 divided by) the sine, cosine and tangent. So
Notice that cosecant is the reciprocal of sine, while from the name you might expect it to be the reciprocal of cosine!
Everything that can be done with these convenience functions can be done by writing things out in full using reciprocals of , and .
Unless you plan to do a great deal of trig and get familiar with working with , and so that you can work with them at speed, it is usually better to stay with , and . Just recognise these functions and be able to convert from and to them in case a question you have to answer is phrased in terms of them.
and because of the definition of cotangent,
Using the definitions and what you already know about sine cos and tan:
Triangles for Reciprocal Trig Functions
The diagrams above show three triangles relating trigonometrical functions. The first one should be familiar to you from the definition of sine and cosine. The remaining two are obtained by (a) dividing all sides by cos θ, and (b) dividing all sides by sin θ.
Because these are all right triangles we can immediately read off variants of the Pythagorean theorem, for these triangles.
The Pythagorean relations can also be derived without the diagrams.
We can divide by to get:
which using the definition of tan and sec is:
Or again starting from
We can instead divide by to get:
which using the definitions for cot and cosec is:
These formula can then be rearranged so that the 1 is on its own on one side of the equals sign, i.e from the tan relation:
and from the cot relation
Remembering the Formulas
It should not be necessary to remember these formulas. None of these formulae is really telling us anything new. You should be able to create them quickly from the relation.
Quick Check of Algebra
It is worth doing a quick check that the formulas are plausible. It is very easy to make a mistake with a sign. Take the last formula involving .
We know that we can make a right triangle with an angle of and sides 1, 1 and so:
So put .
Putting those values into the equation, . It looks OK.
was probably a bad choice as it does not distinguish between and but you should get the general idea about checking the equation is plausible.
Changed Angle Relations
Since cos A = sin (90º-A), it follows (taking reciprocals of both sides) that sec A = cosec (90º-A) or cosec A = sec (90º-A). Also,
it follows that
If you need to, write out the missing step(s) in full.
Fill in the missing expressions on the right: