- #1
cambo86
- 25
- 0
Homework Statement
This is a 2 part question. I'm fine with the first part but the 2nd part I'm struggling with.
The first part asks us to calculate the double integral,
[itex]\int\int[/itex]Dx2dA
for, D = {(x,y)|0≤ x ≤1, x≤ y ≤1}
For this part I got an answer of 1/4.
For the 2nd part we introduce a new coordinate system for D,
x = 1-st, 0≤ s ≤1
y = s, 0≤ t ≤1
The Attempt at a Solution
[itex]\int\int[/itex]Dx2dA
= [itex]\int_0^1\int_0^1[/itex](1-st)2dt ds
= [itex]\int_0^1\int_0^1[/itex](1-2st+s2t2)dt ds
= [itex]\int_0^1[/itex]t-st2+[itex]\frac{1}{3}[/itex]s2t3 ds, from t=0 to t=1
= [itex]\int_0^1[/itex]1-s+[itex]\frac{1}{3}[/itex]s2 ds
= s-[itex]\frac{1}{2}[/itex]s2+[itex]\frac{1}{9}[/itex]s3, from s=0 to s=1
= 1-[itex]\frac{1}{2}[/itex]+[itex]\frac{1}{9}[/itex]
= [itex]\frac{11}{18}[/itex]
I feel like,
dA [itex]\neq[/itex] dt ds
I'm not sure what it equals though. I thought I could use polar coordinates but I don't have a constant radius.