Subjects vector analysis

Greens Theorem 14B0Fe

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1. **Problem:** Verify Green's theorem for the line integral \( \oint_C (3x^2 - 8y^2) \, dx + (4y - 6xy) \, dy \), where \( C \) is the boundary of the region defined by \( y^2 = x \) and \( y = x^2 \). 2. **Green's theorem states:** $$\oint_C P \, dx + Q \, dy = \iint_D \left( \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \right) dA$$ where \( P = 3x^2 - 8y^2 \) and \( Q = 4y - 6xy \). 3. **Calculate partial derivatives:** $$\frac{\partial Q}{\partial x} = \frac{\partial}{\partial x}(4y - 6xy) = -6y$$ $$\frac{\partial P}{\partial y} = \frac{\partial}{\partial y}(3x^2 - 8y^2) = -16y$$ 4. **Compute the integrand:** $$\frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} = -6y - (-16y) = 10y$$ 5. **Set up the double integral over region \( D \):** The region is bounded by \( y^2 = x \) (right boundary) and \( y = x^2 \) (left boundary). Rewrite boundaries for integration in terms of \( y \): - From \( y = 0 \) to \( y = 1 \) (since curves intersect at \( (1,1) \)) - For each \( y \), \( x \) goes from \( y^2 \) to \( \sqrt{y} \) (since \( y = x^2 \) implies \( x = \sqrt{y} \)) 6. **Double integral:** $$\iint_D 10y \, dA = \int_0^1 \int_{y^2}^{\sqrt{y}} 10y \, dx \, dy$$ 7. **Integrate with respect to \( x \):** $$\int_{y^2}^{\sqrt{y}} 10y \, dx = 10y (\sqrt{y} - y^2) = 10y (y^{1/2} - y^2) = 10y^{3/2} - 10y^3$$ 8. **Integrate with respect to \( y \):** $$\int_0^1 (10y^{3/2} - 10y^3) \, dy = 10 \int_0^1 y^{3/2} \, dy - 10 \int_0^1 y^3 \, dy$$ Calculate each integral: $$\int_0^1 y^{3/2} \, dy = \left[ \frac{y^{5/2}}{5/2} \right]_0^1 = \frac{2}{5}$$ $$\int_0^1 y^3 \, dy = \left[ \frac{y^4}{4} \right]_0^1 = \frac{1}{4}$$ 9. **Evaluate:** $$10 \times \frac{2}{5} - 10 \times \frac{1}{4} = 4 - 2.5 = 1.5$$ 10. **Calculate the line integral directly (optional check):** Parametrize the curve \( C \) and compute \( \oint_C P \, dx + Q \, dy \) to verify it equals 1.5. **Final answer:** $$\boxed{1.5}$$ This verifies Green's theorem for the given vector field and region.