Turn on thread page Beta
    • Thread Starter

    Hi everyone, there has been a trig question bothering me for a while from past papers and prelims- we were given the answer along with the question but I don't understand the working. I've gone through the trig chapter in my textbook, and gone through extra notes online relating to higher Maths and still haven't found anything helpful in explaining.

    Here is an example question:

    f(x) = ?3sinx - cosx

    Express f(x) in the form ksin(x-a), where k > 0

    And the worked answer:

    ?3sinx - cosx = ksin(x-a)

    = k [sinxcosa- cosxsina]

    = kcosa . sinx - ksina . cos x

    k cosa = ?3
    ksina = - 1

    therefore k² = 3 + 1
    k = 2

    etc. etc.

    What I don't understand is the point from '= kcosa . sinx - ksina . cos x' onwards.

    Why are cos a and sin x flipped, and how then does this equate to k cos a = ?3 and k sin a = - 1?

    Any help appreciated, cheers.

    I was struggling with the same thing up until the weekend when I found these notes they helped a lot
    Attached Images
  1. File Type: pdf HSN23400.pdf (193.6 KB, 134 views)

    3sinx-cosx Ksin(x-a):ksinxcosa-kcosxsina

    What comes infront of sinx it's 3 so kcosa;3

    same applies for what comes infront of cosx -1 so -ksina:-1 hope tht sort of helped for each type of question just go through same process
    • Thread Starter

    thanks peeps

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
How are you feeling in the run-up to Results Day 2018?
Applying to university
Uni match

Find your perfect match

Our tool will help you find the perfect uni course for you

Make your revision easier

Study Help rules and posting guidelines

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.