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    anm425's Avatar
    anm425 Posts: 1, Reputation: 1
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    #1

    Nov 8, 2010, 02:27 PM
    Prove the identity cos (x+(y-pi/2)) = sin (x+y)...
    1. Prove the identity cos (x+(y-pi/2)) = sin (x+y)

    2. Prove the identity: (5 csc^2 x+4 csc x-1 / cot ^2 x) = (5 csc x-1 / csc x-1)

    3. Prove the identity: cos4u = (cos^2 2u) - (sin^2 2u)

    please help, thank you!

    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Nov 8, 2010, 02:50 PM

    Here are some hints:

    1. If you substitute w for the quantity x+y, you get:

    cos(w-Π/2) = sin(w)

    That should be pretty easy to prove.

    2. Whenever you're asked to prove something involving cotangent, secant, or cosecant, it's almost always a good idea to substitute the primitive functions involving sine and cosine.

    3. Recall that cos(A+B) = cos(A)cos(B)-sin(A)sin(B). From this you can see that cos(2w) = cos(w+w) = cos^2(w) - sin^2(w).

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