Integral question for cos(2z) in complex analysis

In summary, the integral ∫cos(2z)dz from pi/2 to pi/2+i can be evaluated by substituting z=pi/2+i and using Euler's formula, e^(a+bi)=e^a*(cos(b)+i*sin(b)). This simplifies to e^(-2) for the upper limit of the integral and e^(-pi) for the lower limit, giving a final value of e^(-2)-e^(-pi).
  • #1
nate9228
42
0

Homework Statement


Evaluate ∫cos(2z)dz from pi/2 to pi/2+i


Homework Equations





The Attempt at a Solution


I know the cos function is entire, thus independent of path and I just need to evaluate the end points. I also know when you integrate you get (1/2)sin2z. My only question is for finishing the problem, how do I evaluate sin(pi/2+i). Its the i that is throwing me off.
 
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  • #2
sin(z)=(e^(iz)-e^(-iz))/(2i). You know how to do complex exponentials, right?
 
  • #3
Alright I figured it was that formula. So (1/2)sin(2z) becomes (1/4i)(e^(2iz)-e^(-2iz)). Yes I do know how to do complex exponentials , but not very well at a practical level yet. I did take the i out of the denominator of 2+i (using the conjugate) and then after putting into the formula I have (1/4i)(e^((4i[itex]\pi[/itex]+2[itex]\pi[/itex])/5)-e^-((4i[itex]\pi[/itex]+2[itex]\pi[/itex])/5). This I am not so sure how to solve.
 
  • #4
nate9228 said:
Alright I figured it was that formula. So (1/2)sin(2z) becomes (1/4i)(e^(2iz)-e^(-2iz)). Yes I do know how to do complex exponentials , but not very well at a practical level yet. I did take the i out of the denominator of 2+i (using the conjugate) and then after putting into the formula I have (1/4i)(e^((4i[itex]\pi[/itex]+2[itex]\pi[/itex])/5)-e^-((4i[itex]\pi[/itex]+2[itex]\pi[/itex])/5). This I am not so sure how to solve.

Yes, that's the right formula, but I don't see how substituting pi/2+i for z gives what you've got. Where is the '/5' part coming from? And some other stuff doesn't look right. Show what you did. But once you've got it right, then just use Euler's formula, e^(a+bi)=e^a*(cos(b)+i*sin(b)).
 
  • #5
I did (pi/2+i)*(2-i)/(2-i)= (2pi-ipi)/5... specifically the 5 came from (2+i)(2-i)=(2^2)-i^2=5. I then took (2pi-ipi)/5 and plugged it in for z in the formula, which then has it being multiplied by 2i, and 2i*(2pi-ipi)/5= (4ipi+2pi)/5. Sorry for leaving all of that out.
 
  • #6
I did (pi/2+i)*(2-i)/(2-i)= (2pi-ipi)/5... specifically the 5 came from (2+i)(2-i)=(2^2)-i^2=5. I then took (2pi-ipi)/5 and plugged it in for z in the formula, which then has it being multiplied by 2i, and 2i*(2pi-ipi)/5= (4ipi+2pi)/5. Sorry for leaving all of that out.
 
  • #7
nate9228 said:
I did (pi/2+i)*(2-i)/(2-i)= (2pi-ipi)/5... specifically the 5 came from (2+i)(2-i)=(2^2)-i^2=5. I then took (2pi-ipi)/5 and plugged it in for z in the formula, which then has it being multiplied by 2i, and 2i*(2pi-ipi)/5= (4ipi+2pi)/5. Sorry for leaving all of that out.

You are doing some seriously bad complex arithmetic there. (pi/2+i) is not equal to (2pi-i*pi)/5=2pi/5-i*pi/5. The real and imaginary parts are different. I'm not even sure what you think you are doing. Can you explain. Here's part of the right way.

If z=pi/2+i then exp(2iz)=exp(2*i*(pi/2+i))=exp(pi*i-2)=exp(-2)*(cos(pi)+i*sin(pi))=(-exp(2)). That's not so hard, yes? You are putting some kind of extra erroneous step in there.
 
  • #8
Sorry, I was at a loss earlier and did that based on what a friend said because I had no idea. It did make sense to me though so it was my own error. In regards to the arithmetic: I was under the impression when you have i in the denominator, to put the fraction in a+bi form, you multiply the fraction by the conjugate of the bottom. For example, if z= 2/(1+i) you multiply by (1-i)/(1-i) to get (2-2i)/2= 1-i. So I applied that same idea, albeit quite improperly apparently, to pi/(2+i). So basically I multiplied the top and bottom of pi/(2+i) by the conjugate of the denominator, i.e 2-i.

In regards to your answer: Once again, thank you. That's extremely simple. The only thing I'm not understanding at first glance is why does exp(2*i*(pi/2+i))= exp(pi*i-2)? I must say, I find it ironic I am able to prove some of the more challenging theorems, yet I apparently suck at simple complex arithmetic.
 
  • #9
nate9228 said:
Sorry, I was at a loss earlier and did that based on what a friend said because I had no idea. It did make sense to me though so it was my own error. In regards to the arithmetic: I was under the impression when you have i in the denominator, to put the fraction in a+bi form, you multiply the fraction by the conjugate of the bottom. For example, if z= 2/(1+i) you multiply by (1-i)/(1-i) to get (2-2i)/2= 1-i. So I applied that same idea, albeit quite improperly apparently, to pi/(2+i). So basically I multiplied the top and bottom of pi/(2+i) by the conjugate of the denominator, i.e 2-i.

In regards to your answer: Once again, thank you. That's extremely simple. The only thing I'm not understanding at first glance is why does exp(2*i*(pi/2+i))= exp(pi*i-2)? I must say, I find it ironic I am able to prove some of the more challenging theorems, yet I apparently suck at simple complex arithmetic.

To put (a+bi)/(c+di) into rectangular form you multiply BOTH numerator and denominator by the conjugate of the denominator (c-di). But that's not the problem here. Just multiply it out. 2*i*(pi/2+i)=2*i*pi/2+2*i*i=i*pi-2. Maybe being good at the challenging problems is making you overcomplicate the simple ones.
 
  • #10
One last comment.. just because I realized the source of both our confusions as to what the other was doing. I was looking at the limit of integration wrong. Like utterly wrong lol. I thought it was pi divided by the quantity 2+i, when really, as you elucidated, it is (pi/2)+i. That is why I was so confused about the multiplication. I thought it was 2*i*(pi/(2+i))= (2ipi/(2+i)), when clearly its just the normal distributive property, 2*i*(pi/2)+2*i*i. Its amazing what some sleep and the taking another look will reveal.
 
  • #11
nate9228 said:
One last comment.. just because I realized the source of both our confusions as to what the other was doing. I was looking at the limit of integration wrong. Like utterly wrong lol. I thought it was pi divided by the quantity 2+i, when really, as you elucidated, it is (pi/2)+i. That is why I was so confused about the multiplication. I thought it was 2*i*(pi/(2+i))= (2ipi/(2+i)), when clearly its just the normal distributive property, 2*i*(pi/2)+2*i*i. Its amazing what some sleep and the taking another look will reveal.

Ohh. So that's what you were doing. Seemed pretty mysterious, but I probably should have guessed.
 

Related to Integral question for cos(2z) in complex analysis

1. What is the definition of the integral question for cos(2z) in complex analysis?

The integral question for cos(2z) in complex analysis is a mathematical problem that involves finding the integral (or area under the curve) of the complex function cos(2z) over a specific interval.

2. How do you solve the integral question for cos(2z) in complex analysis?

To solve the integral question for cos(2z) in complex analysis, you can use techniques such as substitution, integration by parts, or the Cauchy integral theorem. The specific method used will depend on the complexity of the function and the interval over which the integral is being evaluated.

3. What is the significance of the integral question for cos(2z) in complex analysis?

The integral question for cos(2z) in complex analysis is important because it allows us to calculate the area under a complex function, which can have practical applications in physics, engineering, and other fields. It also helps us understand the behavior of complex functions and their relationship to real functions.

4. Can the integral question for cos(2z) in complex analysis be solved using software?

Yes, the integral question for cos(2z) in complex analysis can be solved using software such as Mathematica, MATLAB, or Maple. These programs have built-in functions and algorithms for solving complex integrals and can provide accurate and efficient solutions.

5. What are some common mistakes to avoid when solving the integral question for cos(2z) in complex analysis?

Some common mistakes to avoid when solving the integral question for cos(2z) in complex analysis include forgetting to account for the complex component of the function, using incorrect substitution or integration techniques, and making errors in the calculations. It is important to carefully check the steps and solutions to ensure accuracy.

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