∫1−521+52(4x3−6x2+7x−1)dx\int_{\frac{1-\sqrt{5}}{2}}^{\frac{1+\sqrt{5}}{2}} (4x^3 - 6x^2 + 7x - 1)dx

Worked solution

1. 通过平移换元将积分区间变换为关于原点对称的形式并化简被积函数

观察积分上下限 1−52\frac{1-\sqrt{5}}{2} 与 1+52\frac{1+\sqrt{5}}{2},它们关于 x=12x = \frac{1}{2} 对称。

令 u=x−12u = x - \frac{1}{2},即 x=u+12x = u + \frac{1}{2},则 dx=dudx = du。
当 x=1−52x = \frac{1-\sqrt{5}}{2} 时,u=−52u = -\frac{\sqrt{5}}{2};
当 x=1+52x = \frac{1+\sqrt{5}}{2} 时,u=52u = \frac{\sqrt{5}}{2}。

将 x=u+12x = u + \frac{1}{2} 代入被积表达式:

4x3−6x2+7x−1=4(u+12)3−6(u+12)2+7(u+12)−1=4(u3+32u2+34u+18)−6(u2+u+14)+7(u+12)−1=4u3+(6−6)u2+(3−6+7)u+(12−32+72−1)=4u3+4u+32\begin{aligned} 4x^3 - 6x^2 + 7x - 1 &= 4\left(u + \frac{1}{2}\right)^3 - 6\left(u + \frac{1}{2}\right)^2 + 7\left(u + \frac{1}{2}\right) - 1 \\ &= 4\left(u^3 + \frac{3}{2}u^2 + \frac{3}{4}u + \frac{1}{8}\right) - 6\left(u^2 + u + \frac{1}{4}\right) + 7\left(u + \frac{1}{2}\right) - 1 \\ &= 4u^3 + (6 - 6)u^2 + (3 - 6 + 7)u + \left(\frac{1}{2} - \frac{3}{2} + \frac{7}{2} - 1\right) \\ &= 4u^3 + 4u + \frac{3}{2} \end{aligned}

2. 利用对称区间上奇函数的积分性质计算定积分

原式转化为:
∫−5252(4u3+4u+32)du\int_{-\frac{\sqrt{5}}{2}}^{\frac{\sqrt{5}}{2}} \left(4u^3 + 4u + \frac{3}{2}\right) du

由于 4u3+4u4u^3 + 4u 为奇函数,其在关于原点对称的区间 [−52,52]\left[-\frac{\sqrt{5}}{2}, \frac{\sqrt{5}}{2}\right] 上的积分为 00:
∫−5252(4u3+4u)du=0\int_{-\frac{\sqrt{5}}{2}}^{\frac{\sqrt{5}}{2}} (4u^3 + 4u) du = 0

因此,积分简化为:
∫−525232du=32[52−(−52)]=32⋅5=352\int_{-\frac{\sqrt{5}}{2}}^{\frac{\sqrt{5}}{2}} \frac{3}{2} du = \frac{3}{2} \left[ \frac{\sqrt{5}}{2} - \left(-\frac{\sqrt{5}}{2}\right) \right] = \frac{3}{2} \cdot \sqrt{5} = \frac{3\sqrt{5}}{2}

Answer

352\frac{3\sqrt{5}}{2}

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