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Math Help - Prove this by whole cube expansion

  1. #1
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    Prove this by whole cube expansion

    prove that (cos theta + i sin theta) ^ 3

    equals

    cos 3theta + i sin 3 theta

    by expanding the whole cube
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  2. #2
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    Re: Prove this by whole cube expansion

    Well, have you tried expanding the cube?
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  3. #3
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    Re: Prove this by whole cube expansion

    yeah i did........ actually we had to it for square and cube, i did for the square but for the cube i can not derive this, i can not prove it using x^3 + y^3 + 3xy^2 + 3yx^2
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  4. #4
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    Re: Prove this by whole cube expansion

    so kindly prove it for me
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  5. #5
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    Re: Prove this by whole cube expansion

    I'll do no such thing, you'll prove it yourself. Start by substituting "cos(theta)" for x and " i sin(theta) " for y. Simplify and collect your real parts and imaginary parts.
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  6. #6
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    Re: Prove this by whole cube expansion

    Prove this by whole cube expansion-demoivers.png
    Thanks from szak1592
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  7. #7
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    Re: Prove this by whole cube expansion

    thanks man, ibdutt, thanks a lot
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