✔ 最佳答案
f(x)= ax - 12, g(x) = 2x + 6, 兩函數圖形交點在x軸上, a= -4 .
設 f(x) 與g(x) 的圖形相交於 x 軸上的點 (c,0)。
把 x = c 及 y = 0 分別代入兩函數中:
ac - 12 = 0 ...... [1]
2c + 6 = 0 ...... [2]
[2]*2 :
4c + 12 = 0 ...... [3]
[1] + [3] :
ac + 4c = 0
a + 4 = 0...(由於 c ≠ 0)
a = -4
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f(x)是一次函數, 且f(-3)= 0, f(1) < 0, f(-5) > 0,f(2) < 0
設 f(x) = ax + b
f(-3) = 0
-3a + b = 0
b = 3a
所以 f(x) = ax + 3a
f(1) < 0
a(1) + 3a < 0
4a < 0
a < 0
f(-5) = a(-5) + 3a
f(-5) = -2a
由於 a < 0,故此 f(-5) = -2a > 0
f(2) = a(2) + 3a
f(2) = 5a
由於 a < 0,故此 f(2) = 5a < 0