Verify: cos3x = 4cos^3 x - 3cosx?

verify as in turn the LEFT INTO RIGHT. or RIGHT INTO LEFT.

Do not alter both to show they are both equal.

Comments

  • cos3x = 4cos^3 x - 3cosx?

    L.H.S. = cos3x = cos(2x+x) = cos2x.cosx - sin2x.sinx

    L.H.S. = (2cos^2x -1)cosx - 2sinx.cosx.sinx

    L.H.S. = 2cos^3x -cosx - 2(1-cos^2x)cosx

    L.H.S. = 2cos^3x -cosx -2cosx + 2cos^3x

    L.H.S. = 4cos^3x - 3cosx = R.H.S. >=========< Q . E . D.

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  • cos(3x) = cos(2x + x) = cos(2x)cos(x) – sin(2x)sin(x) =

    (2cos²x – 1)(cosx) – 2sinx cosx sinx = cosx[(2cos²x – 1) – 2(1 – cos²x)] =

    2cos³x – cosx – 2cosx + 2cos³x = 4cos³x – 3cosx QED

  • By the cosine addition formula, we can write:

    LHS = cos(3x)

    = cos(2x + x)

    = cos(2x)cos(x) - sin(2x)sin(x), since cos(A + B) = cos(A)cos(B) - sin(A)sin(B)

    = cos(x)[2cos^2(x) - 1] - [2sin(x)cos(x)]sin(x), by the double-angle formulas

    = 2cos^3(x) - cos(x) - 2sin^2(x)cos(x), by expanding

    = 2cos^3(x) - cos(x) - 2cos(x)[1 - cos^2(x)], since sin^2(x) + cos^2(x) = 1

    = 2cos^3(x) - cos(x) - [2cos(x) - 2cos^3(x)]

    = 2cos^3(x) - cos(x) - 2cos(x) + 2cos^3(x)

    = 4cos^3(x) - 3cos(x)

    = RHS.

    I hope this helps!

  • cos3x = cos (2x+x)

    = cos (2x)*cos (x) - sin (2x)*sin (x)

    = (2cos^2 (x) - 1)cos (x) - 2sin^2 (x)*cos (x)

    = 2cos^3 (x) - cos (x) - 2(1 - cos^2 (x))cos (x)

    = 2cos^3 (x) - cos (x) - 2cos (x) + 2cos^3 (x)

    = 4cos^3 (x) - 3cos (x)

    = R.H.S

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