Thread: Exam tomorrow.Quick simulataneous equation I'm getting wrong!

Have an exam tomorrow morning and not sure where else to ask at this hour!

Going through some notes and my tutor put:
i1=i2+ (3-8i/4) => i1=3/4 +3i2

I keep getting 2 different answers. Am I right that it should actually be:
i1=3/4-i2? My tutor is prone to mistakes however I doubt my own maths abilities (long story-too much moving around from country to country caused my maths to take a hit often on the easier stuff)!

Would really appreciate your help!

Originally Posted by Joanna Jakma

Have an exam tomorrow morning and not sure where else to ask at this hour!

Going through some notes and my tutor put:
i1=i2+ (3-8i/4) => i1=3/4 +3i2

I keep getting 2 different answers. Am I right that it should actually be:
i1=3/4-i2? My tutor is prone to mistakes however I doubt my own maths abilities (long story-too much moving around from country to country caused my maths to take a hit often on the easier stuff)!

Would really appreciate your help!

By i1 and i2 do you mean $\displaystyle i_1$ and $\displaystyle i_2$?

And here's a multiple choice question:

Does (3-8i/4) mean

A. $\displaystyle \frac{3 - 8i}{4}$

or

B. $\displaystyle 3 - \frac{8i}{4}$ ?

And what is i meant to mean??

You've got to be much clearer. Try using latex.

Thanks!
What's latex? (Unless you're actually talking about the rubber material which i can't imagine!)

By i1 and i2 do you mean i_1 and i_2? -Asnwer is YES

Does (3-8i/4) mean - A. \frac{3 - 8i}{4} YES

And what is i meant to mean? Does it matter what i means?
I'm solving equations for circuit analysis. i is the current in a loop. The equation comes from using loop analysis and KVL. Stuck on just this part.

Thanks a million!

Originally Posted by Joanna Jakma

Thanks!
What's latex? (Unless you're actually talking about the rubber material which i can't imagine!)

By i1 and i2 do you mean i_1 and i_2? -Asnwer is YES

Does (3-8i/4) mean - A. \frac{3 - 8i}{4} YES

And what is i meant to mean? Does it matter what i means?
I'm solving equations for circuit analysis. i is the current in a loop. The equation comes from using loop analysis and KVL. Stuck on just this part.

Thanks a million!

Well since it doesn't appear in i1=3/4 +3i2 (the thing you're trying to get to) and it does appear in i1=i2+ (3-8i/4) (the thing you start with),
yes!! it does matter! Have you just forgotten to include it in i1=3/4 +3i2? Or have you given it a value?

Doh!
Apologies!

"Well since it doesn't appear in i1=3/4 +3i2 (the thing you're trying to get to) and it does appear in i1=i2+ (3-8i/4) (the thing you start with),
yes!! it does matter! Have you just forgotten to include it in i1=3/4 +3i2? Or have you given it a value?"

Yes my mistake!
Starting with i1=i2+ (3-8i2/4)
According to tutors notes this goes to:
i1=3/4 +3i2
I'm getting:
i1=3/4 -i2 instead!

This is what I am trying to solve.
And yes it's (3-8i2) ALL over 4.
(Dealing with 2 currents here i1 and i2)
Hope I'm being clear now.
Sorry about that!

Originally Posted by Joanna Jakma

[snip]
Starting with i1=i2+ (3-8i2/4)
According to tutors notes this goes to:
i1=3/4 +3i2 Mr F says: Wrong.

I'm getting:
i1=3/4 -i2 instead! Mr F says: Correct.
[snip]

If it was i1=i2+ (3 + 8i2/4) then your tutor would be correct.

btw one of the meanings of latex is a mathematics typesetting program. The one used on these boards to get all those equations looking so pretty.

Thanks a million! I got to go kill the teacher! He makes tonnes of mistakes which leave myself and my classmates often uneccessarily puzzled for ages! Thanks for your help!

Originally Posted by Joanna Jakma

Thanks a million! I got to go kill the teacher! He makes tonnes of mistakes which leave myself and my classmates often uneccessarily puzzled for ages! Thanks for your help!

I am not defending a teacher who makes mistakes, but that helps you learn some times!

Good point colby...didn't think about it....though it's not helpful going through tutorial soltuions the days before the exam and puzzling over silly mistakes he's made. I think he could have kept his mistakes to the blackboard!
Just hoping he's not as bad as that when it comes to correcting the papers! Thanks

Everyone makes mistakes. I made one the other day in a maths exam without noticing, until pointed out that i had read the question wrong afterwards.