Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: C3 help

  1. Dec 30, 2007 #1

    Could someone please explain to me the following C3 level question? I would really appreciate any help as I am completely stuck at the moment.

    Given that y = arccos x, -1 <= x <= 1 and 0 <= y <= pi,

    (a) express arcsin x in terms of y.

    I have written down that x = cos y but I do not know where to go from here. It says on the markscheme that:

    x = sin (pi/2 - y) which leads to arcsin x = pi/2 - y

    arcsinx = arcsin cos y

    but I do not understand why this is the case. Could someone please help with this and part (b) of the question also?

    (b) Hence evaluate arccos x + arcsin x. Give your answer in terms of pi.

    Thank you.

  2. jcsd
  3. Dec 30, 2007 #2
    Just draw a picture. We a dealing with a right triangle. Complementary angles add up to pi/2
  4. Dec 30, 2007 #3

    I have drawn a diagram but I still do not understand why x = sin (pi/2 - y). Could someone please explain?

    Thank you for your help.

  5. Dec 30, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper

    cos(y)=sin(pi/2-y) from trig. So if x=cos(y), then also x=sin(pi/2-y). So arccos(x)=y means arcsin(x)=pi/2-y.
  6. Dec 31, 2007 #5
    Thank you for your help!

  7. Dec 31, 2007 #6
    Could someone please draw a diagram for me to compare mine to as I am not sure that I fully understand what it is meant to look like? Could you please label x and y?

    Thank you.

  8. Dec 31, 2007 #7


    User Avatar
    Homework Helper

    Why don't you show your diagram here? Just upload a scan of it or something.
  9. Dec 31, 2007 #8
    hi. if your doing C3 for MEI sylabus this word document helps a lot, its a list of most of the equations they do not give you on any formula sheet. Only thing is that half of the equations are blank, and you have to fill them in, but its good practise. Its really helped me, i'm sitting C3 in a week or so.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook