<非選擇題>(3) 若知 \( f(-1) = 0 \),試求積分 \( \int_{0}^{1} f(x) \, dx \) 之值。(4分)
答案
$ \dfrac{15}{4}$
(3) 已知 \( f(-1)=0 \),求 \( \int_{0}^{1} f(x) \, dx \) 首先,由 (2) 得 \( f'(x) = 3(x + k)^2 \),且 \( f(x) = \frac{1}{3}f'(x)(x + k) = (x + k)^3 \)(展開後與 \( f(x) = x^3 + bx^2 + cx + d \) 對應)。 已知 \( f(-1) = 0 \),代入 \( f(x) = (x + k)^3 \): \[ (-1 + k)^3 = 0 \implies k = 1 \] 因此 \( f(x) = (x + 1)^3 = x^3 + 3x^2 + 3x + 1 \)(對應 \( b=3, c=3, d=1 \))。 計算積分 \( \int_{0}^{1} f(x) \, dx \): \[ \int_{0}^{1} (x^3 + 3x^2 + 3x + 1) \, dx = \left. \left( \frac{x^4}{4} + x^3 + \frac{3x^2}{2} + x \right) \right|_{0}^{1} \] \[ = \frac{1}{4} + 1 + \frac{3}{2} + 1 = \frac{1 + 4 + 6 + 4}{4} = \frac{15}{4} \]
