IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Math => Topic started by: helllife on November 21, 2009, 10:10:31 am

Title: maths doubts....
Post by: helllife on November 21, 2009, 10:10:31 am

question is ...
express this in terms of ratios of acute angles
cos(165)

i know this is simple one...but i just didnt understand how to find it...

Title: Re: maths doubts....
Post by: slvri on November 21, 2009, 10:19:42 am

u know the rule cosx =-cos(180-x)
so cos165=-cos(180-65)=-cos15

Title: Re: maths doubts....
Post by: helllife on November 21, 2009, 11:19:39 am

oh really? is there such a rules like this? can u please give more rule? like about sin?

Title: Re: maths doubts....
Post by: astarmathsandphysics on November 21, 2009, 11:35:55 am

It is cosx=-cos(180-x)
Sinx=Sin(180-x)

Title: Re: maths doubts....
Post by: slvri on November 21, 2009, 11:38:29 am

sinx=sin(180-x)
cosx=-cos(180-x)
tanx=-tan(180-x)
sin(90-x)=cosx
cos(90-x)=sinx
tan(90-x)=1/tanx
sin(-x)=-sinx
cos(-x)=cosx
tan(-x)=-tanx

Title: Re: maths doubts....
Post by: helllife on November 21, 2009, 12:45:08 pm

thank you very much!!!  :)

Title: Re: maths doubts....
Post by: slvri on November 21, 2009, 01:15:05 pm

and also
sin(x+360)=sinx
cos(x+360)=cosx
tan(x+180)=tanx
there are many more but ive forgotten them
and ur welcome ;)

Title: Re: maths doubts....
Post by: helllife on November 21, 2009, 04:05:27 pm

hey what about
sin(180+x), cos(180+x), tan(180+x)??

Title: Re: maths doubts....
Post by: astarmathsandphysics on November 21, 2009, 04:37:49 pm

Will post when i get home. There are some useful formulae.

Title: Re: maths doubts....
Post by: slvri on November 21, 2009, 04:42:52 pm

Quote from: helllife on November 21, 2009, 04:05:27 pm

hey what about
sin(180+x), cos(180+x), tan(180+x)??

oh sory..........tan(180+x)=tanx
and sin(180+x)=-sinx
and cos(180+x)=-cosx

Title: Re: maths doubts....
Post by: astarmathsandphysics on November 21, 2009, 05:24:19 pm

Ok but Sin(180+x)=-Sinx

Title: Re: maths doubts....
Post by: helllife on November 21, 2009, 09:09:26 pm

thank u very much!! i really appreciated it! :)