∫−22(sin⁡x+4−x2)dx=()∫_{-2}^{2}(\sin x+\sqrt{4-x^{2}}){\rm d}x=(\quad)
A. 44
B. 2π2π
C. 4+2π4+2π
D. 88

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Worked solution

1. 利用定积分的性质将积分拆分为两部分

根据定积分的线性性质,可以将原积分拆分为两个部分:
∫−22(sin⁡x+4−x2)dx=∫−22sin⁡xdx+∫−224−x2dx\int_{-2}^{2} (\sin x + \sqrt{4-x^2}) \mathrm{d}x = \int_{-2}^{2} \sin x \mathrm{d}x + \int_{-2}^{2} \sqrt{4-x^2} \mathrm{d}x

2. 计算第一部分奇函数 sin x 在对称区间上的定积分

因为 f(x)=sin⁡xf(x) = \sin x 是定义在 R\mathbb{R} 上的奇函数,且积分区间 [−2,2][-2, 2] 关于原点对称,根据奇函数在对称区间上的积分性质可知:
∫−22sin⁡xdx=0\int_{-2}^{2} \sin x \mathrm{d}x = 0
这里亦可通过微积分基本定理进行验证:
∫−22sin⁡xdx=(−cos⁡x)∣−22=−cos⁡2−(−cos⁡(−2))=0\int_{-2}^{2} \sin x \mathrm{d}x = (-\cos x) \Big|_{-2}^{2} = -\cos 2 - (-\cos(-2)) = 0

3. 利用几何意义计算第二部分被积函数的定积分

对于 ∫−224−x2dx\int_{-2}^{2} \sqrt{4-x^2} \mathrm{d}x,令 y=4−x2y = \sqrt{4-x^2}(其中 y≥0y \ge 0),双方平方整理得 x2+y2=4x^2 + y^2 = 4(且 y≥0y \ge 0)。
这表示以原点为圆心,半径 R=2R = 2 的圆在 xx 轴及上方的半圆。
根据定积分的几何意义,该定积分的值等于此半圆的面积:
S=12πR2=12π×22=2πS = \frac{1}{2} \pi R^2 = \frac{1}{2} \pi \times 2^2 = 2\pi
所以:
∫−224−x2dx=2π\int_{-2}^{2} \sqrt{4-x^2} \mathrm{d}x = 2\pi

4. 求和并确定最终选项

将两部分结果相加:
∫−22(sin⁡x+4−x2)dx=0+2π=2π\int_{-2}^{2} (\sin x + \sqrt{4-x^2}) \mathrm{d}x = 0 + 2\pi = 2\pi
因此,正确答案为 B。

Answer

B

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