Smile 發問於 科學及數學數學 · 1 年前

Maths problem, how to do, thanks?

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  • SC147
    Lv 6
    1 年前
    最愛解答

    Let Jack's original speed = s, original time required = t

    s=30/t - - - - -→ 《s=d/t》

    t=30/s. . . . . . . . . . . . . ①

    new speed = s+3, new time required = t - ½

    s+3 = 30/(t - ½)

    t - ½ = 30/(s+3). . . . . . . . . . . . . ②

    ①-② :

    ½ = 30/s - 30/(s+3)

    1 = 60[1/s - 1/(s+3)]

    1/60 = [(s+3) - s] / s(s+3)

    1/60 = 3 / s(s+3)

    s(s+3) = 180

    s²+3s-180 = 0

    (s-12) (s+15) = 0

    s=12 or s= -15(rejected)

    ∴ original speed, s = 12 km/h

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