Math, please help

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Which of the following are trigonometric identities?
(Can be more then one answer)
tanx cosx cscx = 1


4cosx sinx = 2cosx + 1 - 2sinx

Find all solutions to the equation cosx cos(3x) - sinx sin(3x) = 0 on the interval [0,2π]. (Points : 5)




π/8,9π/8, 15π/8

  • Math, please help -

    tanx cosx cscx
    = sinx/cosx * cosx * 1/sinx
    = 1
    so the first one is an identity

    For questions like this, I use this trick
    pick a weird angle, e.g x = 13.79°
    If that weird angle satisfies your equation, it is "highly likely" that it is an identify.
    ( I know that it does not prove it, but I would put a large sum of money on it)

    2nd one works
    3rd works
    4th does not

    cosx cos(3x) - sinx sin(3x0 = 0
    You should recognize that pattern and get
    cos(x+3x) = 0
    cos (4x) = 0
    4x = π/2 or 4x = 3π/2
    x = π/8 or x = 3π/8
    now the period of cos (4x) = 2π/4 or π/2
    so by adding multiples of π/2 to any answer will give up new answers

    π/8 + π/2 = 5π/8
    5π/8 + π/2 = 9π/8
    9π/8 + π/2 = 13π/8
    etc. (numerator jumping by 4π)
    same for 3π/8 , adding π/2 yields more answers

    3π/8 , 7π/8, 11π/8 , 15π/8 , next one would be > 2π

    looks like you answer #1, with 8 different answers.

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