# Homework Help: I really need some help with these Riemann sum problems

1. Jul 14, 2011

### Jimmy84

1. The problem statement, all variables and given/known data
1.
Express as a sum of riemann and write the integral to express the area of the trapezoid with vertex (0,0) , (1,3) , (3,3) , (5,0).

2.
find the intersection points limited by these equations y = xsquare -3x and y = -2x +3 = 0

3.
the trapezoid with vertex (0,0) , (0,2) , (3,2) and (5,0) spins arround the y axis . Express it as a Riemann sum and write the integral to express the volume of the solid.

2. Relevant equations

3. The attempt at a solution

My biggest problems in problem 1 is that I dont know how to express the area of the trapezoid as an integral or as a function how can it be?

I have looked for this in many math books but they dont mention this at all.

In problem 2 I need to find the intersection points . I tried seting the two equations equal and then solving for x . but it resulted in a quadratic equation and the results werent right.

I need help because I ll have a semestral exam tomorrow about this subject and I really need to master it as much as I can.

2. Jul 14, 2011

### Jimmy84

I already know how to draw the riemann sum and all the details, Im just looking for a way to describe the trapezoid as a function or as many functions so that I could express that function as a riemann sum.

I would appreciate so much any advice.

3. Jul 14, 2011

### Staff: Mentor

Break the problem into three parts, with one integral/Riemann sum for the triangular region on the left, another for the rectangular region in the middle, and a third for the triangular region on the right. For the two triangular regions, the upper boundary is part of a straight line, and you know two points on each line, so you should be able to get the equation of each line.
What equation did you get? Show us what you did.

4. Jul 14, 2011

### Jimmy84

I think I ve now sovled problem one . in another problem im given the vertexs (0,0) , (0,2) , (3,3) and (3,0) . in this case the trapezoid has another shape and I cant do what was exactly done in problem one , my guess is that I should divide it in a square and a traingle but for the square should I use an horizontal rectangle to represent its riemann sum?.

In problem 2 i tried to set the two equatiosn equal, then I tried to solve for x, but i wasent able to factorize so i tried to use the quadratic equation to find x in this way

x square -3x = 2x -3 that is , x square -5x +3 this expresion canot be factorized so
by using the quadratic equation resulted in x = (5 +-square root of 13 )/2

this numbers the answer i got was x=4,30 and x=-0,697 .

If I replace those numbers by x in the original equations and dont get the intersects. How can I find them ?

and in problem 3 should I use the same method of dividing the trpezoid in many areas in order to express the volume of the solid?

Thanks.

5. Jul 14, 2011

### Staff: Mentor

These values are fine.
Use each x value that you got to find the y value at one of the points of intersection. Each intercept has an x-value and a y-value.
I would use the disk method. After you rotate the trapezoid around the y-axis you get a solid that looks like a cone with the top cut off. Each disk of the solid will have a volume of π * (radius of disk)2 * (thickness of disk).

The radius of the disk is the distance from the y axis to the line that goes through (3, 2) and (5, 0).

6. Jul 14, 2011

### Jimmy84

For problem 1 my result was

Integral from 0 to 1 of (3x) dx + integral from 1 to 3 of (3) dx + integral from 3 to 5 of ( (-3/2)x + 15/2 ) dx

I was able to find the intersects for problem number 2 .

for problem number 3 the result should be

pi integral from 0 to 3 of (4) dx + pi integral from 3 to 5 of (-x+5)square dx ?

Are the results right ?

-Also for this problem given the vertexs (0,0) , (0,2) , (3,3) and (3,0) find the integrals. in this case the shape of the trapezoid is diferent I used horizontal rectangles to represent riemanns sum.
The result was:

The integral from 0 to 2 of (2) dy + the integral from 2 to 3 of (3y -6) dy

is this correct?