# The double integral of f over rectangle R and midpoint rule for double integrals

#### Petrus

##### Well-known member
Hello MHB,
I wanted to 'challange' myself with solve a problem with midpoint and rule and the double integral f over the rectangle R.
This is a problem from midpoint.
"Use the Midpoint Rule m=n=2 to estimate the value of the integrab $$\displaystyle \int\int_r(x-3y^2)dA$$, where $$\displaystyle R= {(x,y)| 0\leq x \leq 2, 1 \leq y \leq 2}.$$
Let's start with mid point. I Always start with draw it and the center of those four subrectangles is $$\displaystyle x_1=\frac{1}{2}$$, $$\displaystyle x_2= \frac{3}{2}$$, $$\displaystyle y_1=\frac{5}{4}$$ and $$\displaystyle y_2=\frac{7}{4}$$ and the area of the sub rectangle is $$\displaystyle A= \frac{1}{2}$$ to make it more clear we got $$\displaystyle \frac{1}{2}(f(\frac{1}{2},\frac{5}{4})+f(\frac{1}{2},\frac{7}{4})+f(\frac{3}{2},\frac{5}{4})+f(\frac{3}{2},\frac{7}{4})=-11.875$$
this question is aproxomite same as
$$\displaystyle \int_1^2\int_0^2 x-3y^2 dxdy$$ and that will give result $$\displaystyle -13$$
Now for the most chalange one that I can't solve
I can only solve it with $$\displaystyle m=2$$$$\displaystyle n=1$$(we got two subs in x and 1 in y. cause then we get I start as usually draw it  then we got the area of sub square $$\displaystyle A=1$$ so we got these point $$\displaystyle f(1,2)+f(2,2)$$
and we get result $$\displaystyle -9$$. Now afterwards I type I notice we cant use the rule right? $$\displaystyle x-3y^2=z$$ and for any of our point we will get $$\displaystyle z<0$$ and then we cant use defination. but are my two above correct?

Regards,

#### Sudharaka

##### Well-known member
MHB Math Helper
Hello MHB,
I wanted to 'challange' myself with solve a problem with midpoint and rule and the double integral f over the rectangle R.
This is a problem from midpoint.
"Use the Midpoint Rule m=n=2 to estimate the value of the integrab $$\displaystyle \int\int_r(x-3y^2)dA$$, where $$\displaystyle R= {(x,y)| 0\leq x \leq 2, 1 \leq y \leq 2}.$$
and the center of those four subrectangles is $$\displaystyle x_1=\frac{1}{2}$$, $$\displaystyle x_2= \frac{3}{2}$$, $$\displaystyle y_1=\frac{5}{4}$$ and $$\displaystyle y_2=\frac{7}{4}$$ and the area of the sub rectangle is $$\displaystyle A= \frac{1}{2}$$ to make it more clear we got $$\displaystyle \frac{1}{2}(f(\frac{1}{2},\frac{5}{4})+f(\frac{1}{2},\frac{7}{4})+f(\frac{3}{2},\frac{5}{4})+f(\frac{3}{2},\frac{7}{4})=-11.875$$
this question is aproxomite same as
$$\displaystyle \int_1^2\int_0^2 x-3y^2 dxdy$$ and that will give result $$\displaystyle -13$$
Now for the most chalange one that I can't solve
I can only solve it with $$\displaystyle m=2$$$$\displaystyle n=1$$(we got two subs in x and 1 in y. cause then we get I start as usually draw it View attachment 739 then we got the area of sub square $$\displaystyle A=1$$ so we got these point $$\displaystyle f(1,2)+f(2,2)$$
and we get result $$\displaystyle -9$$. Now afterwards I type I notice we cant use the rule right? $$\displaystyle x-3y^2=z$$ and for any of our point we will get $$\displaystyle z<0$$ and then we cant use defination. but are my two above correct?

Regards,
Hi Petrus, You have used the midpoint rule incorrectly in the second part where you approximate the integral using two rectangles. Note that you should select points which are midpoints of the two rectangles. Then the integral approximates to,

$\iint_R(x-3y^2)\,dA\approx f\left(\frac{1}{2},\,\frac{3}{2}\right)+f\left( \frac{3}{2}\,,\frac{3}{2}\right)$

I don't exactly understand what you meant by,

Now afterwards I type I notice we cant use the rule right? $$\displaystyle x-3y^2=z$$ and for any of our point we will get $$\displaystyle z<0$$ and then we cant use defination.
The integrand can take negative or positive values in the region which it's integrated. There is no problem with that. You might want to check how double integrals are formulated.

#### Petrus

##### Well-known member
Hello,
Yes I understand that. It's because it is continuous function if I got it right, Indeed it says their is for some discontiuos functions but our teacher told us to ignore that cause we will read about it later.

I wanna citat from my book
"If $$\displaystyle f(x,y) \geq 0$$, then the volume V of the solid that lies above the rectangle R and below the surface $$\displaystyle z=f(x,y)$$ is
$$\displaystyle V=\int\int_R f(x,y) dA$$"