# Homework Help: Double intergral

1. Dec 9, 2011

### arl146

1. The problem statement, all variables and given/known data
Consider the heat density function for a metal plate: x + y2 + x2

Find the total heat in the plate given that the plate resides in the region x≥0, y≥0, x2 + y2 ≥ 1

3. The attempt at a solution

i thought it was pretty straightforward. i did a double integral with both limits being 0 to 1 but my professor said the y limit is not to 1. what is the top limit then?

2. Dec 9, 2011

### ehild

Are you sure you copied the last formula correctly? I would mean both x and y going to infinity.

ehild

3. Dec 9, 2011

### arl146

should be less than or equal to 1

4. Dec 9, 2011

### arl146

so is the upper limit of y just sqrt(1-x^2) ?

5. Dec 9, 2011

Correct

6. Dec 9, 2011

### arl146

ok now i cant do the integral..

first step, integrate WRT y. so the function becomes x + (y^3)/3 + x^2
plugging the limits for y in:

[x + [ (1-x^2)^(3/2) / 3 ] + x^2] - x - 0 - x^2

so youre just left with [ (1-x^2)^(3/2) / 3 ]. how do you integrate that WRT x ??

Last edited: Dec 9, 2011
7. Dec 9, 2011

### HallsofIvy

Let $x= sin(\theta)$. That's a pretty standard integral.