题目
1.计算下列曲线积分,并验证格林公式的正确性:(1)oint_(L)(2xy-x^2)dx+(x+y^2)dy,其中L是由抛物线y=x^2和y^2=x所围成的区域的正向边界曲线;
1.计算下列曲线积分,并验证格林公式的正确性:
(1)$\oint_{L}(2xy-x^{2})dx+(x+y^{2})dy$,其中L是由抛物线$y=x^{2}$和$y^{2}=x$所围成的区域的正向边界曲线;
题目解答
答案
为了计算曲线积分$\oint_{L}(2xy-x^{2})dx+(x+y^{2})dy$,其中$L$是由抛物线$y=x^2$和$y^2=x$所围成的区域的正向边界曲线,我们将使用格林公式。格林公式表明,对于一个正向、分段光滑、简单闭合曲线$L$和由$L$所围成的区域$D$,如果$P$和$Q$在包含$D$的开区域内具有连续的偏导数,那么
\[
\oint_{L} P\, dx + Q\, dy = \iint_{D} \left( \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \right) \, dA.
\]
这里,$P(x,y) = 2xy - x^2$和$Q(x,y) = x + y^2$。我们首先计算偏导数$\frac{\partial Q}{\partial x}$和$\frac{\partial P}{\partial y}$:
\[
\frac{\partial Q}{\partial x} = 1, \quad \frac{\partial P}{\partial y} = 2x.
\]
因此,格林公式下的被积函数为
\[
\frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} = 1 - 2x.
\]
接下来,我们需要确定区域$D$。抛物线$y = x^2$和$y^2 = x$的交点通过解方程$y = x^2$和$y^2 = x$找到:
\[
(x^2)^2 = x \implies x^4 = x \implies x(x^3 - 1) = 0 \implies x = 0 \text{ 或 } x = 1.
\]
因此,交点是$(0,0)$和$(1,1)$。区域$D$可以描述为
\[
D = \{(x,y) \mid 0 \leq x \leq 1, x^2 \leq y \leq \sqrt{x}\}.
\]
我们现在可以计算二重积分:
\[
\iint_{D} (1 - 2x) \, dA = \int_{0}^{1} \int_{x^2}^{\sqrt{x}} (1 - 2x) \, dy \, dx.
\]
我们首先对$y$进行积分:
\[
\int_{x^2}^{\sqrt{x}} (1 - 2x) \, dy = (1 - 2x) \left[ y \right]_{x^2}^{\sqrt{x}} = (1 - 2x)(\sqrt{x} - x^2).
\]
接下来,我们对$x$进行积分:
\[
\int_{0}^{1} (1 - 2x)(\sqrt{x} - x^2) \, dx = \int_{0}^{1} \left( \sqrt{x} - x^2 - 2x\sqrt{x} + 2x^3 \right) \, dx.
\]
我们可以将这个积分分为四个独立的积分:
\[
\int_{0}^{1} \sqrt{x} \, dx - \int_{0}^{1} x^2 \, dx - 2 \int_{0}^{1} x\sqrt{x} \, dx + 2 \int_{0}^{1} x^3 \, dx.
\]
我们分别计算每个积分:
\[
\int_{0}^{1} \sqrt{x} \, dx = \int_{0}^{1} x^{1/2} \, dx = \left[ \frac{2}{3} x^{3/2} \right]_{0}^{1} = \frac{2}{3},
\]
\[
\int_{0}^{1} x^2 \, dx = \left[ \frac{1}{3} x^3 \right]_{0}^{1} = \frac{1}{3},
\]
\[
2 \int_{0}^{1} x\sqrt{x} \, dx = 2 \int_{0}^{1} x^{3/2} \, dx = 2 \left[ \frac{2}{5} x^{5/2} \right]_{0}^{1} = \frac{4}{5},
\]
\[
2 \int_{0}^{1} x^3 \, dx = 2 \left[ \frac{1}{4} x^4 \right]_{0}^{1} = \frac{1}{2}.
\]
将所有这些结果组合起来,我们得到
\[
\frac{2}{3} - \frac{1}{3} - \frac{4}{5} + \frac{1}{2} = \frac{20}{30} - \frac{10}{30} - \frac{24}{30} + \frac{15}{30} = \frac{1}{30}.
\]
因此,曲线积分的值为
\[
\boxed{\frac{1}{30}}.
\]