What am I doing wrong when solving this Green's theorem problem?

Click For Summary
SUMMARY

The forum discussion centers on a common mistake in applying Green's theorem to a specific problem involving the line integral of vector fields. The user attempted to solve the integral using the functions \( Q = x \) and \( P = y^2 - 2y \), leading to an incorrect result of \( 3\pi \). The correct evaluation of the integral should have included the expression \( -(\sin t)^3 + \sin t + (\cos t)^2 \), which yields the accurate answer of \( \frac{\pi}{2} \).

PREREQUISITES
  • Understanding of Green's theorem and its applications
  • Familiarity with line integrals and vector fields
  • Knowledge of trigonometric identities and integrals
  • Ability to perform calculus operations involving definite integrals
NEXT STEPS
  • Review the derivation and applications of Green's theorem in vector calculus
  • Practice solving line integrals with various vector fields
  • Study trigonometric integrals and their simplifications
  • Explore common pitfalls in calculus problems and how to avoid them
USEFUL FOR

Students studying calculus, particularly those focusing on vector calculus and Green's theorem, as well as educators looking for examples of common mistakes in integral evaluations.

ainster31
Messages
158
Reaction score
1

Homework Statement



gihQcn6.png


Homework Equations


The Attempt at a Solution



$$Q=x\quad P=y^2-2y\\\oint_C{Pdx+Qdy}\\=\int_{C1}(y^2-2y)dx+xdy+\int_{C_2}(y^2-2y)dx+xdy\\=\int_{-\pi/2}^{\pi/2}(((sint+1)^2-2(sint+1))(-sint))dt+cost(cost)dt\\=\int_{-\pi/2}^{\pi/2}(2sin^2t+4sint+2)\\=3\pi$$

Correct answer is ##\pi/2##.
 
Physics news on Phys.org
Everything is fine except going from second to last step from your answer to the following step.
Integral should be of (- (sint)^3 + sint + (cost)^2) dt
This will give you the correct answer.
 

Similar threads

Replies
3
Views
2K
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
Replies
12
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 21 ·
Replies
21
Views
4K
  • · Replies 6 ·
Replies
6
Views
2K
Replies
32
Views
4K