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DryRun
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Homework Statement
Evaluate [tex]\int\int (2x+1)(x-y)dxdy[/tex] where Ω is the region in the first quadrant of the x-y plane bounded by: y=x, y=x+2, y=2-x^2 and y=4-x^2
The attempt at a solution
This is the graph of the intersecting curves and the shaded area is the region Ω:
http://s2.ipicture.ru/uploads/20120324/RJSv46D4.png
Description of region Ω requires adding 2 parts of the region:
Left-hand side(region A):
For x fixed, y varies from y=2-x^2 to y=x+2
x varies from x=0 to x=1
Right-hand side (region B):
For x fixed, y varies from y=x to y=4-x^2
x varies from x=1 to x=1.56
Now, for the region A:
[tex]\int_0^1 \int_{2-x^2}^{x+2} (2x+1)(x-y)dydx[/tex]
And for region B:
[tex]\int_1^{1.56} \int_x^{4-x^2} (2x+1)(x-y)dydx[/tex]
Therefore,
[tex]\int\int (2x+1)(x-y)dxdy = \int_0^1 \int_{2-x^2}^{x+2} (2x+1)(x-y)dydx + \int_1^{1.56} \int_x^{4-x^2} (2x+1)(x-y)dydx[/tex]
Evaluating double integral for region A:
[tex]\int_0^1 \int_{2-x^2}^{x+2} (2x+1)(x-y)dydx[/tex]
[tex]= \int_0^1 \int_{2-x^2}^{x+2} (2x^2+x-2xy-y)dydx[/tex]
[tex]\int (2x^2+x-2xy-y)dy=2x^2y+xy-xy^2-\frac{y^2}{2}[/tex]
[tex]\int_{2-x^2}^{x+2} (2x^2+x-2xy-y)dy=2x^2(x+2)+x(x+2)-x(x+2)^2-\frac{(x+2)^2}{2}-[2x^2(2-x^2)+x(2-x^2)-x(2-x^2)^2-\frac{(2-x^2)^2}{2}][/tex]
This is the part where the calculations get too crowded. I'm wondering if there's a shortcut or better way of tackling the whole evaluation?
OK, i pushed on as i don't know any other methods:
[tex]\int_{2-x^2}^{x+2} (2x^2+x-2xy-y)dy=x^5+\frac{5x^4}{2}-2x^3-\frac{11x^2}{2}-2x[/tex]
Now,
[tex]\int_0^1 (x^5+\frac{5x^4}{2}-2x^3-\frac{11x^2}{2}-2x)dx=-\frac{8}{3}[/tex]
Evaluating double integral for region B:
[tex]\int_1^{1.56} \int_x^{4-x^2} (2x+1)(x-y)dydx[/tex]
[tex]\int_x^{4-x^2} (2x+1)(x-y)dy=-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8[/tex]
Now,
[tex]\int_1^{1.56} (-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8).dx[/tex]
[tex]\int (-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8).dx=-\frac{x^6}{6}-\frac{x^5}{2}+\frac{7x^4}{4}+\frac{7x^3}{2}-6x^2-8x[/tex]
So,
[tex]\int_1^{1.56} (-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8).dx=-10.45153638-(-\frac{113}{12})=-1.034869713[/tex]
Therefore, the answer is:
[tex]\int\int (2x+1)(x-y)dxdy = -\frac{8}{3}-1.034869713=-3.70153638[/tex]
Evaluate [tex]\int\int (2x+1)(x-y)dxdy[/tex] where Ω is the region in the first quadrant of the x-y plane bounded by: y=x, y=x+2, y=2-x^2 and y=4-x^2
The attempt at a solution
This is the graph of the intersecting curves and the shaded area is the region Ω:
http://s2.ipicture.ru/uploads/20120324/RJSv46D4.png
Description of region Ω requires adding 2 parts of the region:
Left-hand side(region A):
For x fixed, y varies from y=2-x^2 to y=x+2
x varies from x=0 to x=1
Right-hand side (region B):
For x fixed, y varies from y=x to y=4-x^2
x varies from x=1 to x=1.56
Now, for the region A:
[tex]\int_0^1 \int_{2-x^2}^{x+2} (2x+1)(x-y)dydx[/tex]
And for region B:
[tex]\int_1^{1.56} \int_x^{4-x^2} (2x+1)(x-y)dydx[/tex]
Therefore,
[tex]\int\int (2x+1)(x-y)dxdy = \int_0^1 \int_{2-x^2}^{x+2} (2x+1)(x-y)dydx + \int_1^{1.56} \int_x^{4-x^2} (2x+1)(x-y)dydx[/tex]
Evaluating double integral for region A:
[tex]\int_0^1 \int_{2-x^2}^{x+2} (2x+1)(x-y)dydx[/tex]
[tex]= \int_0^1 \int_{2-x^2}^{x+2} (2x^2+x-2xy-y)dydx[/tex]
[tex]\int (2x^2+x-2xy-y)dy=2x^2y+xy-xy^2-\frac{y^2}{2}[/tex]
[tex]\int_{2-x^2}^{x+2} (2x^2+x-2xy-y)dy=2x^2(x+2)+x(x+2)-x(x+2)^2-\frac{(x+2)^2}{2}-[2x^2(2-x^2)+x(2-x^2)-x(2-x^2)^2-\frac{(2-x^2)^2}{2}][/tex]
This is the part where the calculations get too crowded. I'm wondering if there's a shortcut or better way of tackling the whole evaluation?
OK, i pushed on as i don't know any other methods:
[tex]\int_{2-x^2}^{x+2} (2x^2+x-2xy-y)dy=x^5+\frac{5x^4}{2}-2x^3-\frac{11x^2}{2}-2x[/tex]
Now,
[tex]\int_0^1 (x^5+\frac{5x^4}{2}-2x^3-\frac{11x^2}{2}-2x)dx=-\frac{8}{3}[/tex]
Evaluating double integral for region B:
[tex]\int_1^{1.56} \int_x^{4-x^2} (2x+1)(x-y)dydx[/tex]
[tex]\int_x^{4-x^2} (2x+1)(x-y)dy=-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8[/tex]
Now,
[tex]\int_1^{1.56} (-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8).dx[/tex]
[tex]\int (-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8).dx=-\frac{x^6}{6}-\frac{x^5}{2}+\frac{7x^4}{4}+\frac{7x^3}{2}-6x^2-8x[/tex]
So,
[tex]\int_1^{1.56} (-x^5-\frac{5x^4}{2}+7x^3+\frac{21x^2}{2}-12x-8).dx=-10.45153638-(-\frac{113}{12})=-1.034869713[/tex]
Therefore, the answer is:
[tex]\int\int (2x+1)(x-y)dxdy = -\frac{8}{3}-1.034869713=-3.70153638[/tex]
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