Need help finding a radius

[jbjaidee]

Thanks again! I have solved all of my problems, but one is giving me some issues:

-78+j56=

I can solve for angle theta, but not the radius. I think there is some logical reason for this but not sure.

 

[Deleted member 4993]

Thanks again! I have solved all of my problems, but one is giving me some issues:

-78+j56=

I can solve for angle theta, but not the radius. I think there is some logical reason for this but not sure.

Also, they are asking me to construct phasor diagrams for some separate problems using the graphical method for e,f, and g. What does that mean?

Thanks

Can you please show us - how you did solve for angle theta? We can follow your chain-of-thoughts and show you the process for calculating radius?

 

[Dr.Peterson]

Thanks again! I have solved all of my problems, but one is giving me some issues:

-78+j56=

I can solve for angle theta, but not the radius. I think there is some logical reason for this but not sure.

In one of my answers I demonstrated how to find r from x and y using the Pythagorean theorem, r = sqrt(x^2 + y^2). Are you saying you tried that and something went wrong? We'll have to see what you did and what you got in order to diagnose the problem.

 

[jbjaidee]

Yes, I did use the formula you gave me. But I was getting a domain error on my calculator. I removed the exponents and just multiplied the numbers. Here is what I got:

96∠-36°=

 

[Deleted member 4993]

Yes, I did use the formula you gave me. But I was getting a domain error on my calculator. I removed the exponents and just multiplied the numbers. Here is what I got:

96∠-36°=

If you put in the following expression exactly (including parentheses) - you will get correct response:

\(\displaystyle \displaystyle{\sqrt{(-78)^2 +56^2}}\)

I suspect - you did not enclose -78 with a pair of parentheses.

 

[Dr.Peterson]

Yes, I did use the formula you gave me. But I was getting a domain error on my calculator. I removed the exponents and just multiplied the numbers. Here is what I got:

96∠-36°=

I think I mentioned the importance of parentheses when you do this. I asked you to show your work, not just your answer, but I think what you did was sqrt(-78^2 + 56^2), which is wrong.

The error is that "-78^2" means -(78^2) = -6084 because the negative is done before the square. To make the calculator do what you want, you need to explicitly tell it that you want to square the number -78: sqrt((-78)^2 + 56^2). See what you get that way. Actually, when I do these, I know that the negative sign will have no effect because of the squaring, so I don't even bother typing it; I just enter sqrt(78^2 + 56^2). That avoids the chance of a mistake.

Now, what you should have done when you got a domain error (which means it is trying to find the square root of a negative number) is to try again, one step at a time. You would square -78, and either see that the calculator gives a negative result and see what is wrong, or get the positive result you know you should get, add that to the square of 56, and take the square root. When a calculation is complicated, I commonly do it both ways (entering it all at once, and doing one step at a time) to see if I might be entering something wrong. If I get the same answer both times, I'm confident.

Your answer is correct, however (assuming you are expected to round to the nearest integer). I suppose you didn't really mean what you said, "I removed the exponents and just multiplied the numbers"; rather, perhaps you did the squaring by multiplying -78*-78. That's fine, though not necessary.

 

[jbjaidee]

Well I did exactly how you said.

Sqrt((-78)^2+56^2

And I got the correct answer of 96. Thanks, I knew it was something simple like a parentheses.

 

[Deleted member 4993]

Well I did exactly how you said.

Sqrt((-78)^2+56^2

And I got the correct answer of 96. Thanks, I knew it was something simple like a parentheses.

And yet you missed parenthesis again! That should have been written as:

Sqrt((-78)^2+56^2)


​Some calculators allow you to skip the last closing parenthesis - but that is a dangerous habit to form.