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Thread: Trig Expressions and Identities.

  1. March 22nd 2009, 06:45 PM #1
    amanda0603
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    Trig Expressions and Identities.

    Hey all if you could please help i would be so grateful!

    1. Mulitply and simplify-
    cosXsinX(secX-cscX)
    I got 1 by turning it into 1 but Im not 100% sure.

    2. Simplify expression-
    (15tanXcscX-3cscX)/(9tanXcscX-3cscX)

    3. Establish the Indentity
    (1-sin2X)/(sinX-cscX)=-sinX

    4. Establish the identity
    cotX(cotX+tanX)=csc^2X

    5. Establish the indentity
    1- (cos^2X/1+sinX)=sinX

    6. (sinX)/(1+cosX)+(1+cosX)/(sinX)= 2cscX


    I've been working on this forever and cant seem to get these answers, any help is awesome! THANKS SO MUCH
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  2. March 23rd 2009, 04:57 AM #2
    stapel
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    Quote Originally Posted by amanda0603 View Post
    1. Mulitply and simplify-
    cosXsinX(secX-cscX)
    I got 1 by turning it into 1 but Im not 100% sure.
    I'm sorry, but I don't understand what this means...? I mean, yes, you can "turn it into 1" and thus "get" 1, but what was your mathematical reasoning for "turning" something into some value you picked?

    A good way to start is to convert everything into sines and cosines:

    . . . . . \left(\cos(x)\sin(x)\right)\left(\frac{1}{\cos(x)} \, -\, \frac{1}{\sin(x)}\right)

    . . . . . \left(\frac{\cos(x)\sin(x)}{1}\right)\left(\frac{\ sin(x)\, -\, \cos(x)}{\cos(x)\sin(x)}\right)

    How did you go from the above to "1"?

    Quote Originally Posted by amanda0603 View Post
    2. Simplify expression-
    (15tanXcscX-3cscX)/(9tanXcscX-3cscX)
    Factor, and then see what you can do with whatever is left:

    . . . . . \frac{3\csc(x)\left(5\tan(x)\, -\, 1\right)}{3\csc(x)\left(3\tan(x)\, -\, 1\right)}

    . . . . . \frac{5\tan(x)\, -\, 1}{3\tan(x)\, -\, 1}

    I don't know how "simplified" your book wants you to go, so you'll have to decide where to go from the above.

    Quote Originally Posted by amanda0603 View Post
    3. Establish the Indentity
    (1-sin2X)/(sinX-cscX)=-sinX
    The customary technique is to work on the more-complicated side, converting everything to sines and cosines, usually, and applying whatever identities seem useful.

    . . . . . \frac{1\, -\, \sin(2x)}{\sin(x)\, -\, \frac{1}{\sin(x)}}

    . . . . . \frac{\left(1\, -\, 2\sin(x)\cos(x)\right)}{\left(\frac{\sin^2(x)\, -\, 1}{\sin(x)}\right)}

    . . . . . \left(\frac{1\, -\, 2\sin(x)\cos(x)}{1}\right)\left(-\frac{\sin(x)}{\cos^2(x)}\right)

    ...and so forth.

    Quote Originally Posted by amanda0603 View Post
    4. Establish the identity
    cotX(cotX+tanX)=csc^2X
    The left-hand side would appear to be the more-complicated one, so:

    . . . . . \left(\frac{\cos(x)}{\sin(x)}\right)\left(\frac{\c os(x)}{\sin(x)}\, +\, \frac{\sin(x)}{\cos(x)}\right)

    . . . . . \left(\frac{\cos(x)}{\sin(x)}\right)\left(\frac{\c os^2(x)\, +\, \sin^2(x)}{\sin(x)\cos(x)}\right)

    . . . . . \left(\frac{\cos(x)}{\sin(x)}\right)\left(\frac{1} {\sin(x)\cos(x)}\right)

    ...and so forth.

    Quote Originally Posted by amanda0603 View Post
    5. Establish the indentity
    1- (cos^2X/1+sinX)=sinX
    Again, the left-hand side appears to be more complex, so:

    . . . . . 1\, -\, \frac{1\, -\, \sin^2(x)}{1\, +\, \sin(x)}

    . . . . . 1\, -\, \frac{(1\, -\, \sin(x))(1\, +\, \sin(x))}{1\, +\, \sin(x)}

    ...and so forth.

    Quote Originally Posted by amanda0603 View Post
    6. (sinX)/(1+cosX)+(1+cosX)/(sinX)= 2cscX
    What were this instructions on this one? What have you tried? How far have you gotten? Where are you stuck?

    Quote Originally Posted by amanda0603 View Post
    I've been working on this forever
    In future, it might help if you showed that work, so that we can "see" where you're having trouble and then provide intelligent advice. Thank you!
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