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    ok, i've come across this question which i cant seem to solve:

    3tan^2x+4sec^2x=5 and x is obtuse, find sinx exactly

    what i do is change tan^2 into sec^2 - 1 however after doing this then simplifying i always come out with = root (7/8) = sinx
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    tanx=sinx/cosx and secx=1/cosx and cos^2x=1-sin^2x
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    3(sin^2x/(1-sin^2x))+4/(1-sin^2x)=5
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    (Original post by T.P.D-L)
    ok, i've come across this question which i cant seem to solve:

    3tan^2x+4sec^2x=5 and x is obtuse, find sinx exactly

    what i do is change tan^2 into sec^2 - 1 however after doing this then simplifying i always come out with = root (7/8) = sinx
    Changing tan^2 into sec^2 is right, but remember secx is 1/cosx not 1/sinx.

    Also when you square root watch out for "x is obtuse".... Note why it might matter for cos^2x but not sin^2x
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    ok thanks guys so i've got

    3sin^2x/1-sin^2x = 1
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    (Original post by T.P.D-L)
    ok thanks guys so i've got

    3sin^2x/1-sin^2x = 1
    Can you post your working?
    I didn't get that at all :eek:
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    3tan^2x+4sec^2x=5

    3sin^2x/1-sin^2x + 4/1-sin^2x=5

    3sin^2x+4/1-sin^2x = 5

    3sin^2x/1-sin^2x = 1

    o probally went awfully wrong somewhere
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    (Original post by T.P.D-L)
    3tan^2x+4sec^2x=5

    3sin^2x/1-sin^2x + 4/1-sin^2x=5

    3sin^2x+4/1-sin^2x = 5

    3sin^2x/1-sin^2x = 1

    o probally went awfully wrong somewhere
    You've moved the +4 over to the other side when it's in a fraction (it is being divided by 1-sin^2x)
    So you've got 3tan^2x=1 as the last line :nah:

    Actually you are making it more complicated than it has to.
    Try starting with 3(sec^2x-1)+4sec^2x=5
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    so i've ended up with
    7sec^2x=8

    7/cos^2x=8
    7/1-sin^2x=8
    1-sin^2x=8/7

    1-8/7=sin^2x

    rooted = (oot2)/4 yaaay thanks
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    (Original post by T.P.D-L)
    so i've ended up with
    7sec^2x=8

    7/cos^2x=8
    7/1-sin^2x=8
    1-sin^2x=8/7

    1-8/7=sin^2x

    rooted = (oot2)/4 yaaay thanks
    Not quite :p:

    Check what you did between these two lines:
    7/1-sin^2x=8
    1-sin^2x=8/7
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    opps i write it on my paper right just not on here =p

    7/1-sin^2x=8

    1/1-sin^2x=8/7

    1-sin^2x=7/8

    -sin^2x=7/8 - 1

    -sin^2x = -1/8

    sinx = root 1/8

    sinx = root2 over 4
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    (Original post by T.P.D-L)
    opps i write it on my paper right just not on here =p

    7/1-sin^2x=8

    1/1-sin^2x=8/7

    1-sin^2x=7/8

    -sin^2x=7/8 - 1

    -sin^2x = -1/8

    sinx = root 1/8

    sinx = root4 over 2
    Root 1/8 is right
    How does it equal "root4 over 2" though?

    Sry, just trying to make sure it's absolutely right. :o:
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    lol yer another mistype i must of edited my orginal post a second after you quoted from it

    it has been correct now ^^
 
 
 

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