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(1)
f(1) : F(3) = 2 : 1
[a(1) + b] : [a(3) + b] = 2 : 1
(a + b) : (3a + b) = 2 : 1
1(a + b) = 2(3a + b)
a + b = 6a + 2b
b = -5a
b/a = -5

(2)
時間 = 距離/速度

從甲地到乙地需時 = 360 / x 小時
從乙地回甲地需時 = 360 / y 小時

(360 / x) + (360 / y) = 4
360[(1 / x) + (1 / y)] = 4
(1 / x) + (1 / y) = 1 / 90
1 / y = (1 / 90) - (1 / x)
1 / y = (x / 90x) - (90 / 90x)
1 / y = (x - 90) / 90x
y = 90x / (x - 90)

(3)
A - B
= (1 - 5x) - (3x + 7)
= 1 - 5x - 3x - 7
= -8x - 6

已知： -2 < x ≤ 4
-4 ≤ -x < 2
8(-4) ≤ 8(-x) < 8(2)
-32 ≤ -8x < 16
(-32 - 6) ≤ (-8x - 6) < (16 - 6)
-38 ≤ (A - B) < 10

參考資料 wanszeto

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