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posted by on .

please help me with this proving question

prove that

4cos(theta) * cos(theta + 120) * cos(theta - 120) = cos(3theta)


  • math - ,

    Use the cos (A+B) and cos (A-B) formulas to rewrite the left hand side in terms of sin theta and cost theta.

    The product (cos(A+B)*cos(A-B) can be written
    (cos A cos B)^2 - (sinA sinB)^2
    which when B = 120 is
    (1/4)cos^2A -(3/4)sin^2A
    = (1/4)cos^2A -(3/4)(1 - cos^2A)
    = cos^2A - (3/4)
    So the left side becomes
    4 cos^3(theta)-3 cos theta

    Now you work on the right hand side, treating cos (3 theta) as cos (theta + 2 theta) and provde it equals the left hand side.

  • math - ,

    damn thanks

  • math - ,

    I converted the more difficult half of the equation for you. The answer is correct. You should have very little trouble proving the right side is the same.

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