You are Here: Home >< Maths

# c3 trig I need heeeelp! watch

1. Basically I have no idea whatsoever how to answer this question:

prove that cot (A+B) = cot A cot B -1 / cot A + cot B

the answer is in the back of the book but i would love to know how to solve this so i can do it on my own next time. please help someone!
2. tan x = sin x / cos x
so cot x = cos x / sin x
start here
3. What do you know about the identity for and the relationship between and ?
4. (Original post by Goldfishy)
What do you know about the identity for and the relationship between and ?
cot x is 1/tan maybe??
5. @IAmTheChosenOne you could do it like that but that would be long - just use the addition formula and the fact that the usual relation between tan and cot
6. i would start by swapping cot by 1/tan then addition formula, then once simplified change back to cot
7. even if i did replace cot (a+b) with 1/tan i would still be clueless :0
8. (Original post by scholarshipkid)
cot x is 1/tan maybe??
Yep - don't hesitate You can apply this to the the tan addition formula.

If you do manage that, you might like to investigate similar identities. such as or to prove that you know how to do it.
9. so

1/tan(a+b)

where would i go from here?
10. it confuses me because it's a fraction!!
11. okay so now i've got:

tan a + tan b / 1-tan a x tan b

stuckkkk!
12. (Original post by scholarshipkid)
even if i did replace cot (a+b) with 1/tan i would still be clueless :0
If
then
13. (Original post by Goldfishy)
If
then

does cot a + b flip the fraction then?
14. (Original post by scholarshipkid)
does cot a + b flip the fraction then?
Yes because cot is the reciprocal of tan.

But you don't need to use this - you can prove it directly from the fact that cot X = cos X / sin X, so cot (A + B) = cos(A + B) / sin (A + B) and then use the usual sum formulae to expand these and rewrite in terms of cot A and cot B.

Try this and see how you get on.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: September 28, 2010
Today on TSR

### Top unis in Clearing

Tons of places at all these high-ranking unis

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams