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    sally aa's Avatar
    sally aa Posts: 23, Reputation: 1
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    #1

    Jul 14, 2012, 01:13 PM
    cos(pi+theta)cos(pi/2-theta)=-1/2sin2theta
    Prove
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    sally aa Posts: 23, Reputation: 1
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    #2

    Jul 14, 2012, 01:18 PM
    cos(pi+theta)cos(pi/2-theta)=-1/2sin2theta
    Prove
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    Unknown008 Posts: 8,076, Reputation: 723
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    #3

    Jul 15, 2012, 02:57 AM
    Similar to the previous one, you need to make the angle the same as on the right.

    How do you change compound angles to single angles?
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    sally aa Posts: 23, Reputation: 1
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    #4

    Jul 15, 2012, 11:09 AM
    Yah similar
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    #5

    Jul 15, 2012, 11:10 AM
    So? Remember that:
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    sally aa Posts: 23, Reputation: 1
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    #6

    Jul 15, 2012, 11:30 AM
    Yes
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    #7

    Jul 15, 2012, 11:43 AM
    So... if you expand the left side, what do you get?
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    sally aa Posts: 23, Reputation: 1
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    #8

    Jul 15, 2012, 11:52 AM
    (cos pi)(cos theta) - (sin pi)(sin theta)(cospi/2)(costhet) + (sin pi/2)(sin theta)
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    #9

    Jul 15, 2012, 11:56 AM
    Simplify it. There are terms you can, such as
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    sally aa Posts: 23, Reputation: 1
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    #10

    Jul 15, 2012, 12:05 PM
    -1cos(theta)-sin(pi)sin(theta)cos(pi/2)cos(theta)+sin(pi/2)sin(theta)
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    #11

    Jul 15, 2012, 12:07 PM
    Quote Originally Posted by sally aa View Post
    -1cos(theta)-sin(pi)sin(theta)cos(pi/2)cos(theta)+sin(pi/2)sin(theta)
    You wrongly did the substitution...

    Erm... don't you know the others?
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    sally aa Posts: 23, Reputation: 1
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    #12

    Jul 15, 2012, 12:44 PM
    -1cos(theta)-0sin(theta)0cos(theta)+1sin(theta)
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    #13

    Jul 15, 2012, 10:18 PM
    You can simplify this.

    Don't forget the brackets you had. It's supposed to be:
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    #14

    Jul 16, 2012, 10:35 AM
    Okay know what ?
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    #15

    Jul 16, 2012, 10:36 AM
    I told you that you can simplify this...
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    sally aa Posts: 23, Reputation: 1
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    #16

    Jul 16, 2012, 11:27 AM
    is going to be 0-2costheta -0-0 =2cos( theta )
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    #17

    Jul 16, 2012, 11:29 AM
    I don't understand how you could get that...

    I'll take it bit by bit.



    Can be broken down into:
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    sally aa Posts: 23, Reputation: 1
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    #18

    Jul 16, 2012, 11:42 AM
    -costheta-costheta sinthta -sintheta costheta- sine^2 theta like that
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    #19

    Jul 16, 2012, 11:51 AM
    :confused:

    Okay, what is:
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