SeQuEnCeS and SeRiEs: HELP PLZ!

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The first three terms of the sequence -8,x,y,72... form an arithmetic sequence, while the second, third and fourth terms form a geometric sequence. Determine x and y.

  • SeQuEnCeS and SeRiEs: HELP PLZ! -

    y-x = x+8
    y/x = 72/y

    y = 2x+8

    (2x+8)/x = 72/(2x+8)
    4x^2+32x+64 = 72x
    4x^2-40x+64 = 0
    x^2 - 10x + 16 = 0
    (x-2)(x-8) = 0
    x=2,8
    y=12,24

    so, we have

    -8,2,12,72
    or
    -8,8,24,72

  • SeQuEnCeS and SeRiEs: HELP PLZ! -

    thanks:-)

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