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A smart-phone is thrown upwards from the top of a 240-foot building with an initial velocity of 32 feet per second. The height h of the smart-phone after t seconds is given by the quadratic equation h=-16t^2+32t+240. When will the smart-phone hit the ground?

  • math -

    when it hits the ground, h = 0

    so solve:
    16t^2 - 32t - 240 = 0
    t^2 - 2t - 15 = 0
    (t-5)(t+3) = 0
    t = 5 or t = -3, which is not acceptable

    it will hit the ground after 5 seconds

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