How do you answer this question on transforming graphs?

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The graph of y = ab cos (kt), for values of t between 0° and 120°, is drawn on the grid.

Use the graph to find an estimate for the value of

(i)    a,

(ii)   b,

(iii)  k.

The graph is drawn on the last page of the documnet in this link.

http://getrevising.co.uk/resources/specimen_paper_a_for_new_specfication_edexcel_maths

Please help!

Thanks in advance!

Posted Sat 9th June, 2012 @ 18:05 by Dilly

3 Answers

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aaah i don't understand how to do this either?!

Answered Sat 9th June, 2012 @ 19:21 by Terrie
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I think you have to compare it to the graph of y=cost and this graph is a transformation of it.

If you don't know the shape of the graph: http://www.google.co.uk/imgres?q=y%3Dcosx&hl=en&sa=X&biw=1024&bih=653&tbm=isch&prmd=imvns&tbnid=ECGRl2WEXoS7tM:&imgrefurl=http://educationalstationery.com/instructionsbluesrul.htm&docid=-Qr6K5qTRtbeZM&imgurl=http://educationalstationery.com/images/scos.gif&w=313&h=307&ei=_X_UT7LlI5L-8QO92sm3Aw&zoom=1&iact=hc&vpx=94&vpy=198&dur=341&hovh=222&hovw=227&tx=168&ty=124&sig=108553060097168606038&page=2&tbnh=132&tbnw=135&start=15&ndsp=17&ved=1t:429,r:4,s:15,i:149

In this case t is used instead of x.

So, it's been transformed on the y axis by 50 so a-b=50 so a could be 100 and b could be 50. 

It's also been stretched on the x axis by 1/2 . The value of k stretches the graph by 1/k, so k=2.

a=100 b=50 k=2.

Not sure if this is correct, but it's what I would do in an exam.

Sorry if that was wrong, didn't help and confused you :/

If all else fails try looking up the mark scheme or ask/email your teacher :)

Answered Sun 10th June, 2012 @ 12:12 by Hoosierette
  • 0 votes

My maths teacher went through this a couple of weeks ago, and got the answers 50, 50 and 4, but the mark scheme says differently :/

Answered Sun 10th June, 2012 @ 12:59 by Leah